Oliver Letwin condemned Gordon Brown's spending review as "a manifesto for fat government and fake savings" and accused him of presiding over increases in waste and bureaucracy.
The shadow Chancellor told the Commons: "While ministers have been preaching about obesity, their departments have been getting fat on taxpayers' money"
He challenged Mr Brown: "When are you going to accept that what is needed if government is really going to cut out the flab is a complete change of lifestyle? The fact is that fat government is not fit enough to deliver."
To cheers from Conservative backbenchers, Mr Letwin lampooned Mr Brown as the "great wastefinder general" and accused the Chancellor of promising cuts while at the same time delivering large increases in spending.
In a combative performance, Mr Letwin criticised ministers for failing to reduce waste and promised to cut the Government "down to size".
He told MPs: "What we have just heard is a manifesto for fat government and fake savings. The last time the Chancellor told us he was going to make tens of thousands of cuts to the Civil Service was three months ago. Why has he been adding to the numbers of the Civil Service since them?
"What this review really means is more bureaucracy, more targets, more initiatives, more task forces, more centralisation, more regulation, more borrowing and more taxes.
"The Chancellor told us about waste, £21.5bn of waste, and yet this entire spending review is about spending more money. Why is the Chancellor the only person in Britain who thinks the way you waste less is to spend more? People up and down Britain are going to ask themselves the question: Who was it who wasted all this money?"
Mr Letwin said it was a "miracle" that of 88,000 extra jobs in education in the last recorded year only 14,000 were for teachers or teaching assistants.
He said Mr Brown had announced massive cuts in the number of staff at the Department for Work and Pensions two years ago. "Why since then has the number of DWP staff increased by 3,500?" he asked.
Mr Letwin said Mr Brown claimed procurement reforms would save £750m on defence spending. He said: "So why did the defence procurement budget overrun by £3bn last year? That's not just smart, that's definitely a miracle."
He listed a string of areas where he claimed the Government had failed to secure value for its spending. "Why after £305m a year of spending on the New Deal for young people are there still 1 million young people not in work, not in training, not in education?
"Why, after an increase of £30bn on the NHS, are there still a million people on NHS waiting lists? Why, after he has doubled spending on the Home Office, are there still a million violent crimes a year?"
Mr Letwin said Mr Brown was planning to spend £1m a minute and he added: "The only thing that the Chancellor's fat government has delivered is fat taxes."
He asked: "Why will the Chancellor not admit that he has been spending beyond his means and his policies mean third-term tax rises under Labour?"
Mr Letwin declared on behalf of the Conservatives: "We will cut bureaucracy, we will cut inefficiency, we will cut the quangos, we will cut the regulation, we will cut the armies of interferers who do nothing for the people of this country.
And he concluded: "We will cut borrowing, we will cut waiting lists, we will cut failing schools, we will cut crime. We will cut this big fat Labour government down to size."