Sir Philip was asked to look at how efficiently Whitehall departments spend their money.
The Topshop owner claims the Government is not making enough of its buying power.
His view is that the Government fails to make the most of its scale, buying-power and credit rating.
Sir Phillip, worth an estimated £5.1 billion, runs the Arcadia Group of High Street retailers, which also includes Dorothy Perkins and BHS.
But Mark Pack, director at MHP Communications, said the tycoon’s own approach to saving tax sent out a confused message.
Pack said: ‘It seems his appointment and the work of the team have produced some good advice.
‘But his own aggressive approach to minimizing the tax he pays himself sends out a slightly discordant message.
‘People may say the way to save the Government money is to get people like Mr Green to pay more tax.’
George Pascoe-Watson, partner at Portland, said: ‘Sir Philip is clearly an expert who has successfully rationalized the many wings of his business and so can give genuine advice.
‘But he is also a huge name in retailing who is also a pretty well-known figure outside the world of business. It’s a perfect fit for David Cameron’s needs.’
Sir Philip’s report says a lack of a centralised approach means different departments pay hugely different prices for the same items.
Sir Philip said: ‘The conclusion of this review is clear - credit rating and scale in virtually every department has not been used to make government spending efficient.
‘There is no reason why the Government should not be as efficient as any good business.’
Sir Philip was picked to look into government spending by the Prime Minister, David Cameron.
His cabinet is putting the final touches to a programme of deep cuts in public spending, due to be announced next week.
Sir Philip's report has focused on the procurement of goods and services such as computers, travel, print and office supplies and the management of the government's property portfolio.