Deputy Prime Minister John Prescott has launched Labour's first direct attack on Conservative leader David Cameron.
Mr Prescott launched a scathing attack on David Cameron
He told the party's spring conference Mr Cameron was an untrustworthy "chameleon" with "no substance".
In his Blackpool speech, Mr Prescott also appealed for party unity amid the threat of backbench revolts over ID cards and the Terrorism Bill.
Tory party chairman Francis Maude said Mr Prescott's "Punch and Judy" politics was out of tune with the public.
He said: "John Prescott can't seem to come to terms with the fact that David Cameron is changing the Conservative Party and is bringing it back into the mainstream of British politics.
"Instead of indulging in tired Punch and Judy show politics he should be explaining to people why their council tax bills have soared and why he has been putting a brake on improving standards in our schools."
Mr Prescott's call for unity follows a defeat in the Dunfermline and West Fife by-election.
The Liberal Democrats took the Dunfermline and West Fife seat by 1,800 votes, overturning an 11,500 Labour majority at the general election.
Acknowledging the defeat, Mr Prescott said it was the party's responsibility to "persuade the people of Dunfermline and West Fife to come back to Labour at the next general election".
The deputy prime minister accused Mr Cameron of being too quick to change his policies on issues such as patient passports and selection in schools.
He went on to launch a scathing attack on the Conservative leader and his rebranding of the party.
Mr Prescott said: "What we have is Cameron the chameleon. He can change the colour of his skin at will.
'Same old poison'
"But this political animal underneath is Conservative to the core.
"The Tories will come to learn that voters won't trust anyone who writes a right-wing manifesto one month and disowns it a few weeks later.
"That is not courage, that is not conviction - it is pure, naked opportunism.
"They might call themselves compassionate and caring, but we know different. They can change the label any time they like but it is the same old poison in the bottle."
BBC Political Correspondent Norman Smith said Mr Prescott's speech marked Labour taking a "much more direct approach" on its criticism of Mr Cameron, who became the Conservative Party's new leader in December.
Meanwhile Tony Blair will briefly leave his domestic troubles behind as he flies into South Africa for an international summit where world trade and development will be discussed.
Tony Blair used his speech at the conference on Friday to tell delegates that guaranteeing security is as important to Labour as fairness.
The prime minister urged activists to back the government's plans as he prepares for next week's key votes on anti-terror laws and ID cards.