Charles Kennedy has publicly renounced the unofficial tactical voting pact between Labour and the Liberal Democrats in key marginal seats.

The Lib Dem leader has called on all his supporters to back his party, saying a "decisive vote" for the Lib Dems would help put a brake on Tony Blair's power.

"We don't want Tony Blair back with some sort of three-figure majority where he can do what he likes," he said.

Kennedy is said to have been infuriated by Labour claims that a Lib Dem vote could let the Conservatives in by the back door. He described the notion as "utter rubbish".

Relations between the two parties were already strained by the Lib Dems' sharp focus on the prime minister's personal integrity over Iraq.

In a sign of the new hostility between the parties, Tony Blair and Gordon Brown will strongly criticise the Lib Dems' "soft" policy towards drugs.

Writing in the Sun, Blair says: "Their 'law and order' manifesto promises that no one caught with crack, heroin and all other hard drugs would be sent to prison ... As a parent I find such half baked  policies deeply disturbing."

Blair's aides allege the Lib Dem leader reneged on an informal pre-election agreement to make common cause against the Conservatives on immigration and asylum.

"He decided to stay out of the fray on that but then jump on the bandwagon over Iraq. We will go for him," a senior Labour source said.

And when asked about future co-operation with Charles Kennedy, the deputy prime minister John Prescott said in a Guardian interview: "No, no, no bloody compromise with the Liberals, no coalitions, that would be a difficult step after this election."