Downing Street has apologised for e-mails sent by one of Gordon Brown's senior officials which reportedly discussed smearing top Conservatives.
The Daily Telegraph says the e-mails found their way to Paul Staines, writer of the Guido Fawkes blog, who described them as "obscene".
The offending e-mails were sent by Damian McBride, the prime minister's ex-political press officer.
A Number 10 spokesman said the messages were "juvenile and inappropriate".
The Tories said it was "absurd" that advisers were "plotting smear campaigns rather than focusing on how to help people affected by the downturn".
Their author was thought to have sent them to former government spin doctor Derek Draper, who runs a pro-Labour blog.
He branded the idea of an orchestrated Downing Street campaign as "ridiculous".
Mr Draper said he had been sent the comments after canvassing Labour supporters about the prospect of setting up another blog to combat "right wing tittle-tattle" posted on the web.
However, former home secretary Charles Clarke said Mr McBride's actions had brought "shame" to the Labour party and that he should be sacked.
Shouldn't Downing Street officials be concentrating on the challenges we face as a nation?
Chris Grayling, shadow home secretary
The Daily Telegraph said the e-mails included unfounded allegations about Conservative leader David Cameron and shadow chancellor George Osborne.
"The e-mails are intended to be anonymous smears, they are obscene in cases, and would impossible for a newspaper to publish. They're libellous and they're untrue," Mr Staines told the BBC.
"In the e-mails, Damian McBride admits to using 'poetic licence'. He's a civil servant, he's exempt from the restrictions on being impartial and political, he's not exempt from telling the truth."
It is understood Mr McBride had to apologise to colleagues.
Shadow home secretary Chris Grayling described the episode as "ludicrous".
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."
Mr McBride, a special adviser in Downing Street, was removed from his job dealing with the media on a day-to-day basis following the resignation of former Transport Secretary Ruth Kelly in September 2008.
He stayed on in Number 10, and was given responsibility for strategy and planning.
A spokesman for Downing Street 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."
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