Gordon Brown has admitted that Labour faced the fight of its life to win the next general election.
As the party's annual conference got under way in Brighton, he sought to rally ministers, MPs and activists behind a major counter-assault against the Tories.
Writing in a pre-manifesto policy document, the Prime Minister insisted that the choice between Labour and the Tories was now "starker than ever".
Cabinet ministers will use this week's conference to try to portray Labour as the party of "the many", fighting Conservative plans to "cut people adrift".
"We know this will be the fight of our lives," Mr Brown wrote in the foreword to The Choice for Britain. "But we will fight hard because this election is about everything we care about - fairness and responsibility."
His spirited intervention came after former deputy prime minister John Prescott warned there was too much "defeatist" thinking in the party.
Against the backdrop of a consistently double-digit Tory lead in the polls, this week marks Labour's last annual gathering before an election must be held.
Mr Brown, who arrived in Brighton on Saturday afternoon shortly after arriving back in the UK from the G20 summit in Pittsburgh, will be hoping to use the conference as a turning point. He indicated that success for Labour meant targeting the political centre-ground, signalling he would not allow his under-pressure party to veer to the Left.
"We are fighting for a better future for our country," he said. "In the next election campaign, the choices will be starker than ever. To win the fight, we must not only defend our record and achievements - we must show how we are driving forward to meet new challenges."
That included preparing for a "post-crisis" economy, society and politics in the interests of "hard-working people and their families resolutely in the centre-ground of British politics".