user: A total of about 4,000 British troops and French and Belgian civilians died when the ship was bombed in June 1940.
Campaigners including former army head, Gen Lord Dannatt, as well as relatives of those who died, want the wreck to be formally designated a war grave.
The Ministry of Defence said it already had protection under French law.
In a letter to the Times, signatories also including actress Joanna Lumley and author Louis de Bernieres say the government should do more to remember those who died "in one of the forgotten disasters of the war".
They say the suffering of the victims' families is also made worse because they believe some documentary evidence about the disaster is still being withheld.
But the MoD said that all the contemporary documents relating to the sinking were in the public domain.
The letter says: "The sinking may have happened 75 years ago, but to those who still grieve it is not a dim and distant memory.
"The sadness of children orphaned that day who are still alive, remains, and is exacerbated by the fact that Lancastria has never been given the status of official war grave by the British government.
"Furthermore, these relatives do not have a clear understanding of what happened as documentary evidence, they are told, remains unavailable; and possibly will not be revealed until 2040, which is of no comfort; indeed it adds to their distress as the relatives, themselves, will not be alive."
The Lancastria, a Cunard liner in peacetime, was bombed near the port of St Nazaire the day before France surrendered to the Nazis.
It was estimated to be crammed with between 6,000 and 9,000 British troops and French and Belgian civilians and it is believed that more people died than in any other engagement involving British forces in World War Two.
Many were trapped inside the vessel and others struggled to survive in the water that had become thick with oil.
News of the sinking was suppressed on Prime Minister Winston Churchill's direct orders for fear of the effect it might have on morale.
Nearly six weeks later the New York Times broke the story, printing dramatic pictures of the disaster.
There are monuments to the Lancastria in both France and Britain - in St Nazaire, near where she sank, in Glasgow where she was built, one at the National Memorial Arboretum in Staffordshire, as well as a stained-glass window at St Katharine Cree Church in London.
In 2008 the Scottish government commissioned a medal for survivors and descendants of those who were killed.

Sum: The UK government is being urged to do more to commemorate the sinking of the Lancastria in one of the worst maritime disasters of World War Two.


Q: The deputy governor of Ghazni province Mohammad Ali Ahmadi said one militant blew himself up at the prison gates, allowing others to force open cells.
The attackers were well-organised and wearing uniforms, he said.
It is the third time since 2008 that hundreds of Taliban inmates have escaped from an Afghan prison.
Both Taliban spokesman Zabihullah Mujahid and the deputy governor confirmed that three of the attackers had been killed.
The deputy governor said seven police officers were injured and only 80 prisoners were left inside the prison.
Both Mr Ahmadi and the Taliban said the escapees included a number of local Taliban commanders, as well as many other fighters.
Of those freed, the interior ministry said 148 had been jailed for attacks on security forces, while 207 had been convicted for other crimes. Only three prisoners had been recaptured so far.
Mr Ahmadi did not reject the possibility the attackers had had help from inside the jail.
He told the BBC's Mahfouz Zubaide in Kabul that one of the attackers who had been killed was a former inmate who had been released.
The massive overnight jailbreak in Afghanistan's volatile Ghazni province is yet another reminder of the many challenges faced by the Afghan government.
Monday's prison break has happened at a sensitive time for the government, which will complete its first year in office in a week's time, amid questions over its ability to improve security and the economy.
The Taliban will use the attack as a way to boost morale for its fighters, who are divided over succession to founder Mullah Mohammad Omar.
It will also add to concerns among ordinary Afghans about the state's hold on security.
Local officials told the BBC that as the attackers had made their way through the jail, the prisoners did not initially realise the attackers were Taliban because of their uniforms.
The jail is about 5km (three miles) outside Ghazni city, about 120km from the capital Kabul.
In 2011, nearly 500 prisoners - including many Taliban insurgents - escaped from a jail in Kandahar city, after a tunnel hundreds of metres long was dug from the outside.

A: Taliban militants have stormed a jail outside the central Afghan city of Ghazni, killing at least four policemen and freeing more than 350 prisoners.


Article: EU border agency Frontex reported a 70% year-on-year rise in fraudulent documents held by UK-bound migrants.
This was mostly due to Ukrainians using real Polish ID cards and Albanians with Italian and Greek papers, it said.
The Home Office said passports are required at borders to ensure fraudulent documents cannot be used.
Labour's Keith Vaz called for specialist machines across the EU to check IDs.
"The extent of the fraud revealed in this report poses real dangers for our security and undermines our immigration system," Mr Vaz said.
"When a British immigration officer is presented with an Italian or Greek ID, they are clearly unaware of the history or misuse of that document."
The risk analysis from Frontex says a total of 8,373 "document fraudsters" were detected across the EU in 2015.
It does not specify a separate number for the migrants aiming to enter the UK, but a spokeswoman told the BBC the figure had been 3,945 - up from 2,324 in 2014.
The agency said Syrian, Iranian and Chinese were the other nationalities "often" aiming to reach the UK with fraudulent documents.
Overall there is said to have been a "marked increase" in the number of document fraud incidents recorded on movements within the EU's passport-free Schengen area, which does not apply to the UK.
Mr Vaz said: "Countries which are a source of these fraudulent documents, especially those who aspire to EU membership, must tackle this growing problem, and co-operate closely with Europol and other border agencies across the EU.
"This is simply not happening at present."
Frontex also said terrorists were exploiting the migrant crisis to travel unchecked, and highlighted the security risks associated with the abuse of stolen, blank Syrian passports.
There is no EU-wide system of document inspection performance, it said, adding that sophisticated physical and security features of travel documents present "significant challenges for border-control officers".
Justice minister and supporter of the Vote Leave campaign Dominic Raab said EU law was "forcing us to rely on travel documents issued by other EU countries, and prevents us from issuing our own".
Britain Stronger in Europe said the UK has "full control" over borders because it is not part of the Schengen area, while its deal with the EU allows it to co-operate on security issues.
A Home Office spokesman said: "All passengers entering the UK are checked against police, security and immigration watch lists on arrival at the border.
"Where we believe someone poses a risk, Border Force officers can and do refuse them entry.
"Since 2010, we have denied entry to over 100,000 people, including over 6,500 EU nationals."
++++++++++
Summarize:

assistant: A reported rise in migrants aiming to enter Britain using false papers poses real dangers to security, home affairs committee chairman Keith Vaz has said.