ALAN MILBURN last night described how the smear campaign by a sacked No10 advisor had left him “sick to my stomach” , as the Tories demanded an inquiry into what Gordon Brown knew of the scandal.

The Darlington MP and former Cabinet minister spoke out to warn the Prime Minister that the now-notorious emails sent by his close aide Damian McBride had “inflicted huge damage on the Labour Party”.

And he let rip at the long history of “vicious briefing campaigns” by Mr Brown’s allies against his Labour colleagues – victims who are widely thought to include Mr Milburn himself.

The comments came as the Conservatives attempted to raise the heat on the Prime Minister by urging the Cabinet Secretary to investigate who knew of the plan to set up the Red Rag website to air the salacious allegations.

The letter, sent by Francis Maude, the Shadow Minister for the Cabinet Office, read: “Specifically, was the Prime Minister aware of the proposed website or the concept behind it?”

The spotlight turned on Mr Brown after it was revealed that Derek Draper, the other architect of the smear campaign, joined him at Chequers late last year – 12 days after Red Rag was launched.

Tory leader David Cameron also attempted to turn the scandal into an election issue, by insisting only a change of government could drive such dirty tricks out of politics.

Mr Cameron suggested Mr Brown bore responsibility even if he was not told of the specific emails, adding: “He hired these people, he sets the culture, he’s the leader.

“This Government has been in power for too long. They have forgotten what they are there for.”

Mr McBride was forced to resign at the weekend when emails sent from his No10 account emerged, suggesting smears to be posted on the Red Rag site.

They included false allegations about Mr Cameron, Shadow Chancellor George Osborne and backbencher Nadine Dorries.

Yesterday, Mr Milburn – who fought a long feud with Mr Brown before their recent reconciliation – said the smear campaign had “plumbed new depths” in British politics.

He added: “It is morally unacceptable and it has inflicted huge damage on the Labour Party and the Labour Government.

“It is doubly appalling from a Labour Party point of view, firstly because we came into Labour politics because we are committed to certain values and ideas, which we pursue through vigorous debate and not through personal smear.

“Secondly, because what is now clear from media reports is that, for years, it has been members of the Labour Party who have been on the receiving end of vicious briefing campaigns.

“This has got no part to play in Labour politics. I frankly felt sick to my stomach when I read what was being proposed to be said.”

Mr Milburn declined to be drawn on whether he had been a victim of “vicious briefing campaigns”, but his fellow former Blairite Cabinet minister, Stephen Byers, said he bore the scars of Mr McBride’s past activities.

In a newspaper article, Mr Byers wrote: “As a result, I have to admit that I made little effort to suppress a smile when I heard about his enforced departure from Downing Street.”

And he added: “To dismiss the incident as juvenile, which was the first reaction of Downing Street, totally missed the point and failed to recognise the extent of the hurt and offence caused.”

The Conservatives also continued to argue that Mr Brown’s handwritten letters expressing “great regret” fell short of a proper apology – and were unreadable because they were scrawled in a thick marker pen.