John Prescott has opened Labour's annual conference with a warning about the threat from the Conservatives.
The deputy PM says rebuilding party membership is crucial
Speaking in Brighton, the deputy prime minister told party activists it was "tempting" to think the Conservatives were "no threat".
But he said they were the "real enemy" in 100 seats across the country.
Mr Prescott also told supporters they should start working towards a fourth successive election victory and do more to rebuild the party membership.
But Mr Prescott said the party had not celebrated its win in May enough.
He told delegates the Commons majority of 66 was a "luxury" compared to previous Labour administrations.
Mr Prescott's speech, which was switched from its usual end-of-conference slot, was met with a muted response.
He used his opening address to attack David Davis and Ken Clarke - the two frontrunners at the bookies for the Tory leadership.
He accused Mr Davis of "typical Tory hypocrisy", adding that Mr Clarke had presided over "mass unemployment".
As the conference began, protesters gathered outside the Brighton Centre calling for the release of a British resident who is being held as a terrorism suspect by the US at Guantanamo Bay.
Around 300 people waved banners urging the government to uphold the refugee status of 35-year-old Libyan Omar Deghayes.
There was a small group of anti-war protesters.
Mr Prescott also criticised Gate Gourmet, the firm at the centre of a dispute which brought Heathrow airport to a standstill after more than 600 airline catering workers were sacked over a walk-out.
His comments came after union leaders had said the government should take a firmer stand in the dispute.
"The scandalous behaviour we saw with the deplorable treatment by an American company of Gate Gourmet workers is not acceptable in Britain or any modern European country," Mr Prescott said.
Gate Gourmet issued a statement to say it contested the account of the dispute being put forward by the T&G union at the conference.
"The perversion of history being served up by the T&G to the Labour Conference does not in any way reduce our desire to achieve a fair and reasonable solution," it added.
Party chairman Ian McCartney, meanwhile, said he was proud that while Labour governments "are changing the world, the Tories are changing their leader for the fifth time in seven years".
During the opening on Sunday, tributes were paid to the former prime minister, James Callaghan, his wife Audrey and former Cabinet ministers, Robin Cook and Mo Mowlam - all of whom have died this year.
The PM continues to refuse to say when he will step down
"They had so much more to do; so much more to give; so much more to enjoy. They were robbed; we have been deprived," Lord Kinnock told delegates.
Before the conference opened on Sunday, Chancellor Gordon Brown vowed to continue Mr Blair's programme of reform if he becomes the next Labour leader.
The chancellor said there was "no going back" as change was needed to ensure Britain could compete internationally.
The prime minister, who has said he will not stand again as leader, refused to say when he would step down.
Mr Blair has always said his third term as party leader would be "unremittingly new Labour" and is determined to push ahead with change in industry, health, education and welfare.
But he faces a fierce battle with union leaders who have misgivings at the proposed extension of private sector involvement in the public services.