One of Gordon Brown's most senior aides was forced to apologise for sending "juvenile and inappropriate" emails from a Number 10 account.
The private missives, written by special adviser Damian McBride, are said to feature ideas for potential smear stories about senior Conservatives.
They were apparently sent to Derek Draper, who runs a Labour-supporting political blog.
They have now come into the possession of Paul Staines, however, whose Guido Fawkes blog is hostile towards the Government and the Labour Party in general.
A Downing Street spokesman said: "Neither the Prime Minister nor anybody else in Downing Street, except the author, knew anything about any of these private emails.
"The author of these emails has apologised for their juvenile and inappropriate nature and for the embarrassment caused.
"All staff will be reminded of the appropriate use of Number 10 resources."
Mr McBride has been in charge of strategic planning at Downing Street since last October, when he was moved from his position as Mr Brown's political spokesman.
His emails have apparently been touted around newspapers, although none have yet published them. They are thought most likely to see the light of day in a Sunday publication.
According to the Daily Telegraph, the emails feature unfounded claims about Tory leader David Cameron and shadow chancellor George Osborne.
It is unclear how the emails were obtained by Mr Staines.
Tory frontbencher Chris Grayling waded into the row, branding the behaviour "ludicrous".
"I have to say my own view is that this man is one of Gordon Brown's key strategists," he told BBC Radio 4's Today programme. "We are in the middle of the biggest economic crisis for half a century.
"Is this really the best Downing Street can do? Shouldn't they surely be thinking about national issues, shouldn't Downing Street officials be concentrating on the challenges we face as a nation?
"To think that they are sitting in Downing Street putting together ludicrous stories about Conservative MPs that they can put round as tittle tattle just beggars belief really.
"It is a sign of a government that is really getting its priorities in the wrong way."