Iain Duncan Smith runs the risk of appearing "intemperate" as he challenges Tony Blair at the despatch box, a Conservative elder statesman has warned.

Former deputy prime minister Lord Howe says the Tory chief has slipped into the "worst kind of politics" in his weekly attacks on the Labour leader.

"I think that Iain Duncan Smith was right to try and mould himself as 'the quiet man'. When he sought to repudiate stridency he was on the right track," he told The House Magazine.

"But somehow he doesn't follow it through. If you watch his conduct at PMQs, it is inherently strident and slips into the worst kind of politics, which the prime minister does nothing to discourage."

Lord Howe also poured scorn on the "you can't believe a word he says anymore" soundbite deployed by Duncan Smith.

"Iain rightly wants to make politics more realistic and more credible but he doesn't always set about it in the right way," he said.

"As for the catchphrase, I think it's intemperate politics of the least attractive kind. When politicians accuse each other of lying they lower the tone of political discussion.

"Young people in particular are turned off by the failure, by media and political leaders alike, to treat the electorate as grown-ups."

The former Cabinet minister also warns that Duncan Smith must recruit more experienced MPs to his Shadow Cabinet if he is to offer a credible alternative to Labour.

"He could demonstrate that by drawing people within the party to him. When Margaret Thatcher won the leadership of the party she consolidated around herself a widely representative group of frontbenchers and Iain Duncan Smith hasn't been able to do that," said Howe.

"It's not entirely his fault because a lot of former ministers have lost their enthusiasm for 'joining the club' as it were, but somehow the party has to reconsolidate the experience that we have.

"There are a range of people who could give restrained, credible authority and the party would begin to create the impression of a government waiting in the wings, who know what they're about."

Whilst he believes Duncan Smith will retain the leadership, Lord Howe warned that he still has a mountain to climb.

"The party and its leadership have to learn to exist happily with each other. There is no sensible prospect of having another bloodletting leadership election," he added.

"Obviously the appetite for it comes and goes every week and every month, but people need to turn their backs on that and work out how best they can get on and mobilise the campaign that will ensure the realistic prospect of the return of a Conservative government, with a realistic credible programme of action."