Charles Kennedy last night sought to cash in on the defection of ex-Labour MP Brian Sedgemore, claiming it could help the Liberal Democrats to their best General Election performance since the party was founded.

He described the decision of the veteran left-winger to desert Labour after 37 years - 27 of them as an MP - as a "pivotal moment" in the battle for votes on May 5.

The Lib Dem leader also told a rally in Cambridge last night: "We are certainly going to achieve our highest level of voter support for a generation.

"As we enter the last phase of the campaign, I set no ceiling on what we can achieve in this election."

Earlier, Mr Kennedy appealed to disillusioned Labour voters: "The Conservatives self-evidently cannot win this General Election.

"People who want to vote against Tony Blair for a variety of reasons can and should vote Liberal Democrat in that knowledge.

Tony Blair brushed aside Mr Sedgemore's decision to jump ship, saying: "What he does is up to him. He is not even a candidate at the election."

Mr Sedgemore, 68, had already announced he was retiring as an MP after years of defying the Labour leadership on issues such as the Iraq war, tuition fees and foundation hospitals.

But his decision publicly to switch to the Lib Dems reflected the frantic fight for votes in what Labour maintains will be an election hotly contested in marginal seats.

Mr Sedgemore said: "I urge everyone from the centre and left of British politics to give Blair a bloody nose at the General Election and vote for the Liberal Democrats."

The Tories launched a poster yesterday accusing the Prime Minister of lying to take Britain into war.

The move came after the head of the Iraq Survey Group finally confirmed that their hunt for weapons of mass destruction was over.

Saddam Hussein's supposed possession of a WMD arsenal was the official justification for the 2003 war.

Meanwhile, pressure increased on Mr Blair to publish the Attorney General's full advice on the legality of the war, with former Foreign Secretary Robin Cook declaring that the Government's justification for keeping the opinion secret "will not wash".

In his final report, CIA weapons inspector Charles Duelfer said the search for WMD had gone "as far as feasible" and found nothing.

"After more than 18 months, the WMD investigation and debriefing of the WMD-related detainees has been exhausted," he wrote, in a 92-page addendum to the final report he issued last autumn. "As matters now stand, the WMD investigation has gone as far as feasible."

Mr Howard opened a new front in the increasingly personal battle for votes at the weekend when he told voters that Mr Blair "lies to win elections".

Yesterday's poster followed up on the theme, claiming: "If he's prepared to lie to take us to war, he's prepared to lie to win an election."

* Two former County Durham Labour stalwarts have are backing a Liberal Democrat candidate in the General Election.

Jane Hackworth-Young, a former Labour councillor from Mickleton, and former Labour Party official Jim Lowson, from Evenwood, have said they will be voting for Lib Dem candidate Chris Foote Wood for the Bishop Auckland constituency.

Miss Hackworth-Young, a great-great-granddaughter of railway pioneer Timothy Hackworth, and Mr Lowson have blamed the Iraq war, the abolition of community health councils and rising council tax for their switch.

Mr Foote Wood said: "Jane and Jim are typical of thousands of former Labour supporters in the Bishop Auckland constituency who will be voting Lib Dem this time."