DAVID Cameron last night demanded a personal apology from Gordon Brown over the e-mails sent by a top Number 10 adviser discussing smearing the Conservative Party.
A spokeswoman for Mr Cameron said that the Tory leader was "absolutely furious" at the e-mails sent by Damian McBride, the Downing Street spin doctor who quit over the weekend after details of messages he sent began to emerge.
Mr Cameron called on the Prime Minister to give a guarantee that such messages would not be sent again.
Downing Street launched a damage-limitation exercise yesterday, with politicians and advisers doing everything possible to isolate Mr McBride and prevent the fall-out from the affair damaging anybody else, particularly the Prime Minister.
Liam Byrne, the Cabinet Office minister, insisted yesterday that Mr Brown knew nothing of the e-mails sent by Mr McBride.
This claim was reinforced by a No 10 spokesman, who said no-one else in Downing Street knew about the messages, which Mr McBride himself described as "juvenile" and "inappropriate" in his resignation statement.
However, there were suggestions yesterday that Cabinet Office minister Tom Watson had been copied in on the e-mails and had known about them since January.
Mr Watson said he had known nothing of the "completely inappropriate" messages.
Privately, Labour managers expect the effects of this extraordinary row to continue for some time.
Derek Draper, a former spin doctor for Labour who recently returned to politics and has been responsible for co-ordinating a Labour website to counter Conservative-leaning blogs, was the recipient of Mr McBride's e-mails.
He is currently in the Canary Islands and is expected to come under intense pressure over his links to the Labour Party on his return to the UK.
Former Labour home secretary Charles Clarke said the position of Mr Draper should be "looked at", along with that of Charlie Whelan, another former Labour spin doctor, as he had been copied in to the e-mail exchange as well.
Mr McBride resigned after details of his e-mails were published on the "Guido Fawkes" political blog.
Mr McBride had written a series of e-mails to Mr Draper in January suggesting a number of possible avenues to smear Tory politicians. However, all of his claims were unfounded.
One suggested there might be "embarrassing" pictures of George Osborne, the shadow Chancellor; another suggested raising questions about whether or not Mr Cameron had an "embarrassing illness"; and another made lurid allegations about Conservative backbencher, Nadine Dorries.
Ms Dorries confirmed she was among four Tories who were discussed in the e-mails, and called for the Prime Minister to say sorry.
"I would also like to know how Gordon Brown would feel if Conservative Central Office wrote such disgusting lies about his wife, Sarah Brown," she said.
Chris Grayling, the shadow home secretary, claimed that the e-mails showed a "structured plan" to publish "blatant lies" about opposition MPs.
He said: "This was right at the very heart of our government. It's a sign of something absolutely rotten at the heart of Gordon Brown's Downing Street."
He added: "The real question now is: was Mr McBride the only person involved in all of this?"
Mr McBride – who has been close to Mr Brown since his Treasury days – said in a statement that he was "shocked and appalled" at the way the e-mails had been used, and regretted any embarrassment caused to the government.
The row could hardly be worse for Mr Brown. Not only has the Prime Minister worked closely with Mr McBride for several years, but he has also insisted that his administration will not be as obsessed with spin as its predecessor.
John McDonnell, a Labour backbencher, has called for an investigation to find out who was involved.
He demanded that the Prime Minister act decisively by launching an independent inquiry.
"Smear tactics like this are not the Labour way," he said. "They drag the Labour Party into the gutter. They just add further to the undermining of the belief that Labour Party supporters have placed in our party."
Nationalist MP Angus MacNeil said: "This whole affair leaves a real stench in Downing Street, and Gordon Brown must say what he knew and when.
"At such a time of economic uncertainty, Gordon Brown must put an end to Labour's dark and dirty politics, and focus on helping households and high streets."
The behind-the-scenes figures unmasked by political scandal for internet age
PAUL STAINES, 41, is better known as Guido Fawkes. The Irishman started his blog in 2004 after being declared bankrupt; the right-leaning site now gets about 120,000 hits a month.
Mr Staines's site has become the best-known, and most successful, Westminster political blog and has claimed at least one notable scalp when Peter Hain resigned from the Cabinet after the blog revealed undeclared donations.
DEREK DRAPER, 41, has been at the heart of the New Labour operation – on and off – since Tony Blair took over leadership of the Labour Party in 1994.
Mr Draper was a top spin doctor in the first year of the Labour government until he was recorded in 1998 boasting to an undercover reporter: "There are 17 people who count. To say that I am intimate with every one of them is the understatement of the century."
This led to a cash-for-access scandal and he withdrew from frontline politics. He later had a breakdown and then retrained as a psychotherapist.
A year ago Draper returned to politics to establish a Labour blog to take on the Conservative blogosphere.
DAMIAN MCBRIDE, 35, was a former Treasury spin doctor who made the move with his master to Number 10.
Last year's debacle at Labour conference, when the departure of Ruth Kelly from the Cabinet was leaked at a 3am briefing in a bar, led to him being moved to a behind-the-scenes strategic post.
As the spin doctor himself said: "We all know that when a backroom adviser becomes the story, their position becomes untenable."
Osborne the cross-dresser and other lies
DAMIAN McBride suggested several possible stories to ensure the New Labour "attack" website got off to a good start.
None was true.
• The first he described as a "solid investigative story" and suggested it "may be a good one to use early". It involved a gay Tory MP and allegations he was promoting his companion's business interests in the Commons.
"He invites MPs to a reception at the Commons – courtesy of the taxpayer – to promote (the retailer's) new fair-trade range of products."
The other suggestions for stories, Mr McBride admitted, were "gossipy and mainly intended to destabilise the Tories".
• Mr McBride seized on the admission by David Cameron in an interview once that, when a student, he attended a clinic for sexually transmitted diseases.
Mr McBride suggested that Mr Cameron should publish his financial and medical records – though there is no suggestion that the Tory leader ever had treatment at the clinic.
• The next subject of a possible smear was the shadow chancellor, George Osborne. Mr McBride suggested hinting that Mr Osborne might have embarrassing pictures from his university days.
"Embarrassing photos have followed George Osborne around throughout his career ... but he knows that the most embarrassing photos from his past have yet to emerge."
Mr McBride then suggested spreading rumours that pictures existed of Osborne "posing in bra, knickers and suspenders" and "with his face blacked up".
• Mr McBride also asked: "Why are friends of George Osborne letting it be known that his wife Frances has been feeling emotionally fragile?"
• In another, later e-mail, Mr McBride returned to the theme of Mr Osborne and embarrassing pictures, and also made some unfounded allegations about Nadine Dorries, a backbench Conservative MP.