text: Sir Philip, owner of clothing retailer Arcadia Group, will examine expenditure from the past three years to try to identify potential savings.
  He told the BBC more centralised buying by government departments might help.
  But the appointment was criticised by the GMB union - who said that Sir Philip's wife, the direct owner of Arcadia, lives in a tax haven.
  Sir Philip owns more than 2,000 shops in the UK, including BHS and Topshop, which are estimated to make up some 12% of the nation's clothing retail market.
  His conclusions will inform the wider Comprehensive Spending Review due in October.
  Sir Philip Green is one of the UK's most successful retailers.
  With a personal fortune of more than £4bn, he owns the Arcadia Group, whose fashion chains include Topshop, Burton, Dorothy Perkins, Evans and Miss Selfridge.
  In 2009 Arcadia made pre-tax profits of £213.6m, 13% higher than the previous year.
  Sir Philip also owns department store chain BHS, whose fortunes he has transformed since he purchased the then ailing company in 2000.
  The government will be hoping that Sir Philip can transfer his renowned ability to trim costs from the private sector to the civil service.
  He will also bring a proven record in procurement, and aims to reduce the cost of the government's purchasing contracts.
  However, despite Sir Philip's successes, his appointment may still raise eyebrows.
  His wife Tina is the direct owner of Arcadia, and she is officially a resident of Monaco. This enabled her to gain a tax-free £1.2bn dividend in 2005.
  Sir Philip also made headlines in 2004 when he made a failed hostile takeover bid for Marks and Spencer.
  He told BBC Radio 4's Today programme he had "no idea" how much could be saved yet, but added: "We need to get ourselves focused on the big spending, where is the money actually being spent.
  "I think in the past, occasionally having had discussions on this, there is not central procurement.
  "So the thought process of us having an individual buyer in each one of our stores without centralising the purchasing, will give you some idea, hopefully, of the opportunity.
  "But it's about process as well, so we need to understand the process, how it's done, who does what and how we can quicken all of that up."
  But his appointment was criticised by Paul Kenny, head of the GMB union, who  pointed out that Sir Philip's wife - the named owner of Arcadia - lives in the tax haven of Monaco.
  He said: "If we were seeking advice about marketing or selling clothes he is someone you might ask. This is about the health, education and care of millions of our fellow citizens - not about importing cheap clothing."
  And the Public and Commercial Services Union, which represents civil servants, said: "It's not that surprising that the millionaires in the cabinet have appointed a billionaire to say that their cuts, which will devastate communities in the UK, are 'fair'."
  But Cabinet Office Minister Francis Maude defended Sir Philip's appointment, saying he had a "sharp eye for detail" and could guide the government on contracts and leases signed up to in future.
  "He's shown how he can turn around big complex businesses. Government is a huge complex organisation, and while it's not the same as a business, a lot of the same disciplines are needed," he added.
  Asked about the tax status of his wife, Sir Philip told the BBC: "My wife's not a tax exile - my family do not live in the United Kingdom, it's somewhat different."
  He added: "We do pay all our tax in Britain. I think we have paid over the last five years some £300-400m in taxes on profits that have been made on our company.
  "I'm a UK taxpayer, I work here every week, we employ 45,000 people in the UK and we have got a £500m payroll."
  Sir Philip will be supported by a team of civil servants and report to Mr Maude and Chief Treasury Secretary Danny Alexander - who said the review would help the government "totally re-think" the way it spends public money.
target: Billionaire Topshop owner Sir Philip Green is to lead a review of government spending - amid criticism from unions.