Alistair Darling has warned that Britain faces its toughest spending cuts for 20 years if Labour continues in office.

The Chancellor, indicating a dramatic shift in his party’s election strategy, tells The Times today that severe spending restraints are “non-negotiable” if he is to bring down the £178 billion budget deficit.

The remarks suggest a big victory for Mr Darling and Lord Mandelson after the attempt to bring down the Prime Minister. Gordon Brown has apparently been dissuaded by two of his most powerful Cabinet colleagues from adopting a simplistic “investment versus cuts” election campaign associated with his close adviser Ed Balls.

Two days after the botched putsch, The Times can also reveal that Mr Brown will ask voters to give him “a full second term” at the polls. A Labour official released the unequivocal statement to avoid unwanted speculation in coming months.

Mr Darling and Lord Mandelson were upbeat yesterday after urging the Prime Minister to show the country and the money markets that Labour was more serious than the Conservative Party about slashing public debt and enhancing its credibility with the middle classes.

“My priority is to get borrowing down. Once recovery is established we have to act,” the Chancellor said.

“The next spending review will be the toughest we have had for 20 years . . . to me, cutting the borrowing was never negotiable. Gordon accepts that, he knows that.”

Lord Mandelson said: “We have got to win people back to our side. We are a national party, a party that represents people in every part of the United Kingdom, and we are going to offer policies that will benefit people in every part of the United Kingdom.”

Their remarks reinforce the impression that ministers used Mr Brown’s temporary weakness on Wednesday afternoon to warn him that he had to be more collegiate to retain their support and to ensure that the party again fights the election as “new Labour”.

Mr Darling admits that when he met Mr Brown late on Wednesday afternoon they talked about the Budget. His interview suggests that he was happy with the outcome — yesterday the Cabinet had its first meeting since the “botched putsch” and not one minister mentioned it.

The Chancellor’s language about cuts in his interview would have been unthinkable a few weeks ago. He says that Labour will be more realistic than the Tories, who still have to explain how they will pay for their concessions on marriage tax allowances and inheritance tax.

“We are saying to people there are some things that matter [which] we need to protect. But the next spending review will be tough. There will be programmes that need to be cut. It will be the toughest for 20 years,” Mr Darling said.

Officials made plain that the Comprehensive Spending Review to which Mr Darling referred would happen after the election, although he indicated that more details would be spelt out then.

His certainty that there will be a Budget, which cannot take place under law before March, is further confirmation that the election is likely to take place on May 6.

Mr Darling said that Labour will “absolutely not” entertain a core-vote strategy at the election. “Peter said the same thing this week,” he said, referring to Lord Mandelson.

“Narrow appeals to one particular group or another simply don’t work. Gordon was involved in the start of the new Labour process. One of the reasons Tony Blair became leader was because Gordon had taken unpopular decisions to junk some of the old Labour stuff we had accumulated. It is blindingly obvious you cannot win elections if you cannot capture the middle ground.”

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