Gordon Brown has faced down critics who have called on him to stand down ahead of the coming general election by declaring: "I do not roll over".
As Labour began its final annual conference before the election, which must take place within nine months, Mr Brown acknowledged the party faces "the fight of our lives".
But he insisted that it was not too late to win back voters who have switched to David Cameron's Conservatives by persuading them that a Tory government would harm public services, cost jobs and prolong the recession.
Despite a News of the World poll putting the Tories 14 points clear of Labour, Mr Brown insisted that the voters were still "suspending judgment" on his handling of the economic crisis and the MPs' expenses scandal and might shift back to Labour as recovery from recession became more established.
He used the first day of the conference in Brighton to try to launch a fight-back, as several of his close allies admitted Labour had not been performing well enough.
Chancellor Alistair Darling said the party did not appear to have "fire in our bellies", while Cabinet colleague Peter Hain said unless Labour started fighting to win it was facing "a really bad defeat".
The Prime Minister sought to regain the policy initiative, announcing plans for a Fiscal Responsibility Bill - to commit future governments to cutting Britain's £175 billion deficit - and a Business and Financial Services Bill, to clamp down on bankers' bonuses.
But Mr Brown was again forced to fend off questions about his health during an interview on BBC1's Andrew Marr Show, denying suggestions - which have been circulating on the internet - that he is dependent on prescription painkillers or other pills.
Mr Brown said neither he nor his party had ever been daunted by the difficulties that arise while in power.
"A setback can either be a challenge which means it is an opportunity to do something better, or you can roll over," he said. "I do not roll over. A setback for me is a challenge, an opportunity to learn, of course, if you have made mistakes, and to do things better. That is what it's all about."