The Conservatives are refusing to accept assurances from No 10 that one of Gordon Brown's closest ministerial allies knew nothing of plans to create an "attack blog" to propagate political smear campaigns in the run-up to the next election.

News of the planned smear campaign emerged at the weekend and forced the immediate resignation of Damian McBride, the Prime Minister's former head of strategy, who cooked up a series of slurs against leading Tories - including David Cameron.

This morning Sir Gus O'Donnell, the Cabinet Secretary, turned down a call from the Conservatives for him to investigate whether Tom Watson, the Cabinet Office minister, had known of the planned Red Rag website which Mr McBride discussed with a former Labour spin doctor, Derek Draper.

In the letter to Francis Maude, who shadows the Cabinet Office, Sir Gus said: "Tom Watson has made his own position clear." He added that the Prime Minister had made clear "that he has been assured that no minister or political adviser other than Damian McBride had any knowledge of, or involvement in, the e-mails".

But Tory sources said today that the party was not willing to let matters lie and Mr Maude, a former party chairman, would reply to Sir Gus this afternoon to complain that there were too many questions still unanswered.

Mr Maude also wants an assurance from Sir Gus, the head of the Civil Service, that he himself has been able to examine Mr McBride's email records to make sure that nobody else was involved.

"He says that there's been assurances but from whom and on what basis," one senior source said. "The impression Dowing Street is giving is that this was a rogue initiative. We are not convinced of that fact."

Independent investigations into members of the government would normally be carried out by the ministerial adviser on interests, currently Sir Philip Mawer, but such a probe can only be triggered by the Prime Minister.

In his letter, Sir Gus said that the actions of Damian McBride, who as a "special adviser" was officially a civil servant, fell "far short" of the public's expectations.

He warned that any repeat of the behaviour by special advisers would result in automatic dismissal.

"What happened constituted a clear and serious breach of the Code of Conduct for Special Advisers," Sir Gus wrote. "It cannot and has not been tolerated."

The Cabinet Secretary confirmed that Mr McBride had not received severance pay, adding: "As the PM has made clear, the events reported over the past weekend were not acceptable, and fell far short of the high standards the public has a right to expect."

Sir Gus said he had written to all permanent secretaries - the chief civil servant in each Whitehall department - to inform them of an update to the code of conduct.

"In particular, under this strengthened guidance, it has been made specifically clear that special advisers will automatically be dismissed if they are ever found to be preparing and disseminating inappropriate material," he said.

Special advisers are to be required to sign an undertaking that they are aware of the new guidance.

The Tory backbencher, Nadine Dorries, who was among those targeted by Mr McBride's smears, stepped up calls for Mr Brown to make a full apology.

Ms Dorries, MP for Mid Bedfordshire, hit out at both the tone and the content of a handwritten letter sent to her by Mr Brown which expressed "great regret", but stopped short of a personal apology.

Meanwhile, Mr McBride's co-conspirator in the planned smear campaign, Derek Draper, revealed today he was considering stepping down from running LabourList, a Labour-supporting website.

He also told The Guardian that he "deeply regretted" his part in the smear emails.

"I should not have responded to Damian's e-mail as I did. I should have said sorry, that is wrong, I will have nothing to do with it," he said.

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