Nick Clegg will veto George Osborne's demands for a two-year freeze in most state benefits from next April and a further £10bn of welfare cuts.
In an interview with The Independent, an unrepentant Mr Clegg said he had no regrets about his dramatic public apology for breaking his promise to vote against a rise in university tuition fees.
"Of course people are going to sneer and say it is not good enough. I genuinely thought what we did was wrong and I should apologise for it," he said. Insisting he will lead the Liberal Democrats "through and beyond the next election", he accused his critics calling for a different leader of "losing their nerve."
The Deputy Prime Minister revealed he will block the Treasury's demand for more cuts before the 2015 election to compensate for lower-than-expected growth. "Not a penny more, not a penny less," he declared.
He disclosed that he will limit the scope of the government-wide spending review due next year to a single financial year, 2015-16, a shorter period than the Treasury wants. He will approve "no cuts" for the post-election period unless Mr Osborne brings in a wealth tax or mansion tax on homes worth more than £2m.
Interviewed on the eve of the Liberal Democrat conference in Brighton starting today, Mr Clegg:
l backed a move at the conference to censure the Chancellor for obstructing the drive towards green energy, saying the industry needs more certainty from the Government to attract investors;
l demanded the Chancellor announce in his Budget next spring that the threshold at which people start paying income tax will rise from £9,205 to £10,000;
l vowed to block the Conservatives' plan to bring in regional pay for public sector workers if, as he expects, it would "widen the north-south divide".
Mr Osborne wants to announce a blanket two-year freeze on state benefits from next April in his autumn statement in December.
Mr Clegg's intervention means this cannot happen, although the Liberal Democrat leader did not rule out an extended freeze on child benefit, aleady held flat for three years.
He said: "You cannot fill the remainder of the black hole from the wealthy alone. But that is where you start, and you work down. You don't start with the bottom and then work up. That is just wrong."
The Liberal Democrat leader said: "It is not realistic to assume you cannot have any further cuts and savings. Where they fall is an entirely different matter. "The Conservatives appear to be saying they want it all to fall on welfare. That's totally unacceptable to me. They are not going to take all of that £10bn out of welfare.
"I am not saying you can leave welfare untouched, because it is a third of total public spending.
"But the idea that you ask welfare to take all of the strain is something I will not allow to happen," he added.