Plans to change the way union members donate to Labour could "end rather than mend" relations, says the leader of the UK's third largest union, the GMB.
Paul Kenny told the BBC that Labour leader Ed Miliband's "bold move" was "as close as you can get" to ending the link between Labour and the unions.
Mr Miliband wants union members to have to actively opt in to join Labour, rather than being automatically affiliated as part of union membership.
The union says its affiliation fees paid to Labour could drop from around £2m a year to less than £1m as a result of the changes.
Mr Kenny, its general secretary, said the union would decide whether to ballot its members on whether they still want to give money to Labour in September - meaning the funding cut could happen as early as next year.
Under the current system members are given the chance to vote on a union's political fund every 10 years.
Media captionEd Miliband: "I do not want any individual to be paying money to the Labour Party in affiliation fees unless they have deliberately chosen to do so"
But Mr Miliband said on Tuesday he would end the automatic "affiliation" fee paid to Labour by three million union members and people should not pay Labour any fees "unless they have deliberately chosen to do so".
The changes would have "massive financial implications" for the party, he acknowledged, but could raise its membership from the current 200,000 to a "far higher number".
The announcement came after Unite, one of the party's biggest donors, was accused of signing up its members to Labour in Falkirk - some without their knowledge - in an effort to get its preferred candidate selected to fight the next election.
"This is an absolute watershed moment - the changes they are about to do will fundamentally change the party's relationship with trade unions and their members," Mr Kenny said.
He told BBC Radio 4's Today programme that collective affiliation and individual membership were two "entirely different things".
The union would be "lucky" if 10% of the 400,000 members who are currently affiliated to Labour on a collective basis would choose to sign up as individual members, he said.
"We have been trying to encourage our members to join Labour for a very long time - through magazines, meetings and our conference. I have to say we have not been knocked down in a rush.
"The jump will be from currently affiliating the union to argue for people on political policies with the Labour Party to individual membership of the party. Lots of people agree with the first but I am not sure that many people agree with the second.
Mr Kenny criticised the language he said had been used by some Labour politicians in recent weeks describing it as "disappointing and insulting" and said he would not be surprised if some members wanted to disaffiliate from Labour entirely.
Gerry Morrissey, the general secretary of media and entertainment union Bectu, which is also affiliated to Labour said he believed Mr Miliband's proposals were "totally unnecessary", as members could choose to opt out of the 3p a week payment on their membership forms.
"I don't know what percentage of ours would opt in but if you looked at normal ballot procedures etc, I wouldn't be expecting much more than 20-25% of people to be opting in," he told BBC Radio 4's World at One. His party has donated more than £50,000 to Labour since the general election.
Unite general secretary Len McCluskey has said the union's political levy - worth £8m a year to Labour - would "stay as it is" and he is happy to discuss the proposals with Labour as the current relationship between the party and the unions is not satisfactory.
Mr Miliband's initiative has been welcomed by senior figures in the party, including Tony Blair, who said he wished he had done it when he was party leader.
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Sebastian Vettel is "proud" of becoming the first driver to win eight consecutive races in a single season.
The Red Bull driver broke a record held by Michael Schumacher and Alberto Ascari with victory in the US GP.
"Numbers do not make me jump in the car but we realised we had done it and it makes you very proud," Vettel said.
"You realise what it means to look back at those names. People look back and talk about certain drivers. One day people might talk about our time."
The German has won every race since Formula 1 returned from its summer break at the Belgian Grand Prix at the end of August.
He said the achievement, which he described as "incredible" would not fully sink in until Monday morning.
"It's always the day after usually you feel invincible - don't get me wrong," said Vettel, who tied up his fourth consecutive title in India three races ago.
"It is great feeling. You have a grin on your face, unless you have a hangover. It is nice feeling, Sunday things happen very quick and it is bit harder to realise.
"It is one of those records that was not meant to be broken ever and it's very difficult to understand we actually did it.
"It is down to team effort. Everyone is willing to push. This morning at eight o'clock the car was in pieces - we had the gearbox on the floor we had some concern about reliability and the guys were trying to fix it and it ran fine in the race.
"Since the summer break I don't know what we have all done, but we need to remember for future years should we require a little bit more than just good performance.
"I keep saying there is no guarantee for the next race, for the next race and equally for the year after. We had a phenomenal run that last five years. You never know what is coming next. You have to enjoy today and look forward to tomorrow."
Red Bull team boss Christian Horner said: "When we reflect what we have achieved it is quite remarkable, the levels Sebastian has reached, the accuracy and consistency he has driven with is quite astounding.
"He was pretty emotional at the end of the race because he has beaten a record of one of his idols, who it seemed unlikely that kind of record would be beaten.
"To win eight consecutive races, to have won every race since July [when Lewis Hamilton won the Hungarian Grand Prix] is mind-blowing, especially against the quality of opposition we're up against. It'll take a little while for it to sink in with the whole team."
In their thousands the American crowds thronged for the exit gates oblivious to the unbridled celebrations erupting inside the ropes.
European players were hugging, jumping for joy, faces bubbling and blubbering in ecstasy and astonishment. They'd just completed the most unlikely of comebacks and the Ryder Cup still belonged to them.
A lone figure stood arms waving, face beaming. He looked up at the BBC Radio 5 live commentary box and we exchanged grins. It was Bernard Gallacher who'd been giving our listeners wonderful insight during the drama that unfolded over the three days at Medinah.
Yes, there was an undoubted sense of triumph about the Scotsman and why not? He is a three-time former captain and victory over the Americans means an awful lot to him.
But there was more to that expression of joy. There was a sense that he was celebrating witnessing something very, very special. "The best ever Ryder Cup," he later said.
And it is that vision of a veteran golfer, who has seen so much of what the game has offered over the last half century, enjoying those moments with such relish that provides my abiding golfing memory of 2012.
This could easily have been the year when no one noticed golf. Such were the sumptuous sporting spoils the game could easily have been buried by the dramas elsewhere.
But golf more than held its own throughout the past 12 months. It was a truly vintage year which, from a European point of view, reached its peak in the outskirts of Chicago in early autumn.
Surging back from 10-4 down, inspired by a seemingly possessed Ian Poulter, Europe gained a victory that was as unlikely as it was dramatic. Justin Rose rammed home that monster putt on the 17th and his opponent Phil Mickelson sportingly applauded.
In truth, he probably still thought the Americans would win back the precious trophy but that Rose putt was part of an unstoppable European tide.
It was almost as though the hand of a smiling Seve Ballesteros was at work from above to help out his old mucker Jose Maria Olazabal, whose captaincy for so long looked like ending in uncharacteristically meek defeat.
Instead we were left with Tiger Woods conceding an eminently missable putt to Francesco Molinari to hand Europe their win after Martin Kaymer had held his nerve to ensure the trophy would be retained.
Miracle or meltdown? Probably a bit of both and how the US rued sticking the 17th pin on the front right of the green. It was a location their players were ill-equipped to find when the pressure was at its greatest.
And this stunning Ryder Cup was so in keeping with the rest of the year. Rory McIlroy had surged to the top of the world rankings with victory in West Palm Beach to lay down the marker in early spring.
Then came Augusta and Bubba Watson's outrageous wedge that flew sideways from the trees on the 10th to claim play-off victory over Louis Oosthuizen. The South African had earlier benefitted from an albatross two at the second, holing out from the fairway on that steeply downhill par five.
Luke Donald successfully defended the PGA title at Wentworth. We thought it had set him up perfectly for a tilt at the US Open. But his challenge never made it out of the blocks and McIlroy's title defence also foundered with a poor missed cut.
Lee Westwood challenged at the Olympic Club but faded when his ball freakishly failed to tumble from the high branches of a tree in the final round.
That US Open was a hard watch - the course set up was brutal and Webb Simpson proved last man standing.
Ernie Els made it a belly putter major double the next month at July. The South African took full advantage after Adam Scott's broomhandle lost its magic and his drive at the 72nd found sand.
Scott's capitulation, squandering a four-stroke lead over the closing holes, was sad to see but a fourth major for the popular Els was fitting compensation.
Little did we know then that McIlroy was about to turn round a season that was proving a struggle in the big events of high summer.
"It all clicked when he finished fifth in Akron," his coach Michael Bannon recalled. That was the week before the year's final major, the US PGA at Kiawah Island.
The Northern Ireland youngster arrived at the Ocean Course and, looking out from the locker room, loved what he could see. He was ready to win and win big.
McIlroy romped to an eight-stroke victory, the same winning margin as when he won the 2011 US Open. He had won two majors at a younger age than Tiger Woods managed.
Woods had a decent year with three wins, but was repeatedly undone with poor weekend performances after promising starts. It suggests an underlying mental frailty that will need to be addressed in 2013.
McIlroy powered to two wins in September's PGA Tour play-off series and wrapped up his fifth win at the season ending DP World Tour Championship. He'd already won the money lists on both sides of the Atlantic.
Although player of the year in Europe and America, McIlroy might have been remembered more for missing his singles Ryder Cup tee-off time had it not been for a state trooper rushing him to the course after failing to realise when he should have been starting his round.
A man 30 years older than McIlroy, Roger Chapman, also deserves remembering at the end of 2012. The former European Tour journeyman beat star-studded fields to win the Senior PGA and US Open.
He called them "life changing" wins and he was accorded honorary life membership of the European Tour. "I've just paid my subs," he joked when he was given the award. That's my favourite quote of the year.
Tribute should also be paid to Tegwen Matthews's Great Britain and Ireland team which won the Curtis Cup at Nairn. It was the first time the US had been beaten in 16 years and meant that all the transatlantic trophies were held on this side of the pond.
Europe's amazing win at Medinah ensured this remains the case as we head into the new year. No wonder Gallacher and the rest of European golf is smiling at the moment.
Connacht have received a boost after their highly-rated scrum-half Kieran Marmion agreed a two-year contract extension with the Irish province.
Marmion, 21, was named in the Ireland squad for last summer's tour of North America although he remains uncapped.
Marmion has represented Ireland at youth level and is now on the fringes of the country's senior squad.
He is currently behind Conor Murray, Eoin Reddan and Isaac Boss in the Irish scrum-half pecking order but that may not remain the case for too much longer.
After spending only one year in the Connacht Academy, Marmion made his first competitive start for the province in last season's Pro12 opener against Cardiff Blues and went on to start every game during the campaign.
Indeed, Marmion hasn't missed a game for Connacht since his debut 14 months ago and he has scored three tries in his 36 matches.
"It's a real boost that he sees his continued development and future with the club he started his professional career with," added Lam.
"From day one, Kieran has impressed me with his attitude, work ethic and incredible maturity and I have no doubt that he will go on to achieve great things within Connacht and Irish rugby."
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