Iain Duncan Smith has called on the prime minister to apologise to the survivors of the Paddington crash over his "attempt to discredit them".

Despite an onslaught from the Tory frontbench, Tony Blair denied that there was any attempt by spin doctors to smear Pam Warren, the former chairman of the survivors' group, who criticised Stephen Byers over the winding up of Railtrack.

He said he "endorsed entirely" the Department of Transport's position.

"There was never any attempt to discredit Pam Warren," he told MPs. "However I stand entirely by the apology given by the Department of Transport."

Duncan Smith accused the prime minister of using "weasel words" to mask wrongdoing.

"Will he now give Mrs Warren and the other survivors the apology they deserve," said the Tory leader.

Blair said the Paddington group had been misled into believing that the Labour Party was attempting to discredit them.

"She [Warren] had been told that there had been an attempt to find out or dig up dirt - that was simply not the case. There was no inquiry, indeed, ever made about Pam Warren."

But Duncan Smith refused to accept Blair's reassurance, calling on him to apologise "without equivocation and without qualification".

"Those are weasel words," said the Tory leader. "This is the same man who promised to bring in a new politics of honesty and trust.

"And yet when any member of the public, whether it's Rose Addis or Pam Warren or anybody else, criticises him, his government throws the weight of its machine to investigate their private lives and to crush them."

Blair insisted "there was never any email attempting to dig dirt on Pam Warren". "If he has such an email he can now produce it," he demanded.

Lib Dem leader, Charles Kennedy, also called on the prime minister to tackle the sleaze claims surrounding ministerial special advisers and spin doctors.

The Lib Dem leader quizzed Blair on the case of an MoD adviser who joined a PR company with extensive defence contracts within days of leaving government.

Andrew Hood, a former special adviser to defence secretary, Geoff Hoon, joined the PR firm Brunswick within weeks of quitting Whitehall.

Kennedy demanded to know whether Blair was "satisfied that the spirit and the substance" of the rules governing transfers had been observed.

"The MOD have processes which apply to the whole of government, which they carried out, the permanent secretary certified it, and it was done in consultation with the Cabinet Office in the normal way," said the prime minister.

From the public's point of view, said Kennedy, "special advisers are subject to exactly the same rules...as are senior civil servants".

"Quite frankly, for a lot of people in the country this case and the explanations which attach to it, with the government, stretch credulity," he told MPs.

"When is the prime minister going to take further action to clean up this state of affairs?"

Blair continued to stress that the "right and proper" rules had been applied in the "normal way".

And he insisted that special advisers continued to play an important role in government.

"Rather than dealing with perception... isn't it about time we dealt with the reality.

"The reality is that special advisers, for both governments, Labour or Conservative, do a perfectly good job.

"I fully support the special adviser system and I think it would be deeply unfortunate if we got rid of it."