Mr Blunkett quit his cabinet post with immediate effect after admitting his office had fast-tracked a visa application for his ex-lover’s Filipina nanny.

The country’s top law enforcer, blind from birth, insisted he hadn’t been told officials were writing to the immigration department over the case of nanny Leoncia Casalme.

“However, whether or not I asked for any action to be taken is irrelevant to the inference that can be drawn,” Blunkett said.

He’d enjoyed Downing Street’s support over the matter, but his position became more uncertain after he criticised a string of Cabinet colleagues in a new biography.

Mr Blunkett always denied he had intervened to speed up the application for a British visa, but conceded an inquiry had uncovered evidence his officials did so instead.

He said he’d been told about “a fax and an exchange of e-mails” that showed the visa application had been fast-tracked.

An inquiry was established to probe newspaper claims Mr Blunkett intervened improperly to help get a British visa for Casalme, who worked at the time for his then lover, married magazine publisher Kimberly Quinn.

The allegations reportedly originated with Quinn, from whom Blunkett had separated in August amid acrimony.

Mr Blunkett always denied he’d intervened to speed up the application, but conceded the inquiry had discovered a series of e-mails about the visa application between his office and immigration officials

“Given I have no recollection of issuing instructions to deal with the application, but only to continuing the elimination of the backlog in general, the easy thing would be to hide behind my officials,” he said.

“I will not do such a thing. In no way is my office or any individual within the department to blame for what happened.”

The resignation robs Blair of one of his key lieutenants only months before a general election expected in May next year.

In accepting the resignation, Prime Minister Blair described him as “a force for good in British politics”.

Mr Blunkett, whose remit included immigration, policing and anti-terrorism, among other areas, was one of the government’s most respected ministers, especially among Britain’s powerful right-wing tabloid press.

However, tough policies such as locking up foreign terrorism suspects without trial prompted fierce criticism from human rights groups.

Born into poverty in 1947, Mr Blunkett was blind from birth due to a genetic disorder and fought against terrible odds to go to university and then into politics.

However his image has been severely dented in recent weeks, both by the nanny allegations and by lurid newspaper revelations about his three-year relationship with Quinn.

Mr Blunkett is currently fighting a bitter legal battle to establish his paternity over Quinn’s two-year-old son.

He’s also reportedly seeking to claim paternity for the unborn child Quinn, who is seven months pregnant, is carrying.

In an emotional interview with the BBC, he suggested he’d been willing to sacrifice his political career to pursue his legal claim

“He will want to know not just that his father actually cared enough about him to sacrifice his career, but he will want to know, I hope, that his mother has some regret.”