Iain Duncan Smith will today portray Tony Blair as a dodgy conman whose word cannot be taken at face value.

In a speech to be delivered in the New Labour citadel of Millbank Tower, the Tory leader will step up his campaign against the integrity of the prime minister.

Addressing party supporters he will say that "there should be no third chance for Tony Blair's third way".

"Tony Blair lacks the conviction to bring about the real changes we need. He sings the song of change as if change were an end in itself," Duncan Smith is expected to say.

"He says delivery is on its way, but he said the same thing in 1999, 2000, 2001 and again last year.

"There is a sense of drift and decay about this government - which no number of relaunches will reverse. It has run out of steam.

"The prime minister is running out of friends. And the British people have had enough."

The Conservatives believe that Blair's credibility is now the Labour Party's Achilles heel.

Tax increases combined with the controversy surrounding the "dodgy dossier" have hit the prime minister's personal standing.

A survey published earlier this week revealed that two in three voters distrust the premier.

Likening the prime minister to used-car dealer Arthur Daley, Duncan Smith will say Blair's word on tax and public services cannot be trusted.

The attack, which comes on the eve of the summer recess, forms part of a concerted Tory campaign.

This week the Conservatives launched the latest poster offensive, claiming Labour wants to push higher rate tax up to 61 pence in the pound.

Last week the party characterised Blair as Pinocchio, complete with growing nose, in what appears to be an increasingly personal assault on the Labour leader.

Duncan Smith will add: "The reason is that Tony Blair and Gordon Brown are the Arthur Daleys of British politics - the dodgiest of used-car salesmen, trying to flog a clapped-out, broken-down model for change by slapping on a new coat of paint."

"They tried to con middle Britain into believing that their policies were Conservative - and that you could have a Conservative government without the Conservative Party.

"They dumped some unpopular old Labour policies. But although the language changed, their instincts did not."