The head of the Civil Service has apparently refused calls by the Tories to investigate whether a Labour minister was involved in a dirty tricks operation that cost the job of a senior aide to Gordon Brown.

Sir Gus O'Donnell, the Cabinet Secretary, was asked to probe whether Tom Watson, the Cabinet Office minister, was aware plans for a smear website called Red Rag.

But he appeared to have rejected the request in a letter to Francis Maude, the Shadow Minister for the Cabinet Office. Sir Gus instead appeared to indicate he was relying on statements made by the minister instead.

He said: "Tom Watson has made his own position clear."

Yesterday Mr Watson said through solicitors Carter-Ruck, that he had "no involvement in or knowledge of" the proposals to set up an attack website called Red Rag.

Sir Gus said 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 Sir Gus does not endorse or reject Mr Watson's statement.

Independent investigations into members of the government would normally be carried the ministerial adviser on interests, currently Sir Philip Mawer, which can only be triggered by the Prime Minister. Such a probe appears not to have been asked for.

In his letter, Sir Gus said that the actions of Damian McBride, Mr Brown's former head of strategy and planning, fell "far short" of the public's expectations.

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

Mr McBride, a longstanding adviser to the Prime Minister, resigned at the weekend after it emerged he had written e-mails making unfounded personal allegations about senior Tories.

"What happened constituted a clear and serious breach of the Code of Conduct for Special Advisers.

"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 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, 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 letter sent to her by Mr Brown which expressed regret, but stopped short of him saying sorry.

It was sent after Mr McBride quit after admitting he wrote "juvenile and inappropriate" emails from his Downing Street account to former spin doctor Derek Draper.

Mr Draper revealed today he was considering stepping down from running LabourList, a Labour-supporting website.

He also told a newspaper 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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