Gordon Brown was accused of putting up a “smokescreen” on Monday after he sought to distance himself from the activities of his disgraced former aide by urging tightening of the civil service rules.

Mr Brown insisted he and other ministers had been unaware Damian McBride, his head of strategy, was proposing a smear campaign against senior Tory MPs.

The prime minister wrote personal letters to all those mentioned in the e-mails. He also wrote to Sir Gus O’Donnell, cabinet secretary, calling for changes to the code of conduct for special advisers to prevent such behaviour in future.

That would include an explicit assurance that dissemination of inappropriate material by advisers would mean automatic dismissal.

David Cameron, the Tory leader, welcomed the fact Mr Brown had at last “recognised the gravity of what has been taking place in Downing Street”.

But one Conservative source said the proposed changes to the code of conduct were a “smokescreen” given that the existing rules were strong enough.

There was also anger in Tory circles the prime minister had offered “regret” rather than apologies and had described the smears as “unsubstantiated claims” rather than as lies.

The decision met incredulity in Westminster where Mr McBride was long known to be a ferocious spinner for his political master.

Michael Meacher, a former Labour minister, said Mr Brown had promised to end spin, secret briefings and dirt-digging when he became prime minister in 2007. “All these elements of the Blair era have actually got worse,” said Mr Meacher.

Mr McBride, one of Gordon Brown’s closest aides, was forced to resign at the weekend after publication of e-mails he wrote to Mr Draper about ideas for a new website to be called Red Rag.

Mr Draper was already operating a mainstream blog called LabourList with the help of several senior Labour figures.

Mr McBride had proposed the new blog should smear senior Tories by a host of unfounded allegations. The plan was abandoned after second thoughts by the two men.

Mr Brown’s decision last night was made after Tories pressed government to disclose who else may have been involved.

Francis Maude, shadow minister for the Cabinet Office, called for information about whether Mr Brown was aware of the proposed website and who else was party to the e-mail exchanges.

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