A key member of Tony Blair'a (search) Cabinet and the man in charge of legislation on crime and terrorism in Britain resigned after acknowledging that his department fast-tracked a visa for his former lover's nanny.

David Blunkett's (search) departure Wednesday was a blow for Blair, who had strongly backed him and confidently asserted he would be cleared of wrongdoing. Blunkett was Britain's top law enforcement officer and had been establishing a new system of national identity cards.

Blunkett said he felt he had to step down because his actions led to preferential treatment for a residence visa for a Filipina nanny employed by his former lover, American magazine publisher Kimberly Quinn (search).

"I believe these issues would never have been raised had I not decided in September that I could not walk away from my youngest son," said Blunkett, who has gone to court to establish that he fathered Quinn's 2-year-old son.

"Any perception of this application being speeded up requires me to take responsibility," he told reporters. He denied that he had been dishonest about his role.

Britain's only blind legislator, the divorced Blunkett gained some sympathy for his shattered love affair. His three-year relationship with the married Quinn soured over the summer, and Blunkett has gone to court in an attempt to gain access to the 2-year-old son. Quinn, currently expecting another child whose paternity is the subject of intense media speculation, denies the toddler is Blunkett's son.

Blunkett has suggested Quinn leaked details of their relationship and the "Nannygate" claim to the media as a form of revenge.

Blunkett's place in the Cabinet became shakier with the revelation that he made caustic comments about colleagues to the author of a newly published biography.

Blair on Wednesday appointed Education Secretary Charles Clarke as Home Secretary to replace Blunkett. Clarke, 54, will be replaced in the education post by Ruth Kelly, currently Cabinet Office minister, Downing Street said. Home Secretary is a key Cabinet post with responsibility for law and order, immigration and counterterrorism.

Blunkett's problems have been a distraction and embarrassment for Blair's government since the affair was revealed in August, but it got worse this month with the disclosure of his low opinion of some colleagues.

Quinn's nanny, Leoncia Casalme, had been told by the Home Office that it might take a year to process her visa application. Quinn brought that to Blunkett's attention, and he said he in turn showed the letter to officials to stress that such delays were unacceptable.

That letter got into the system, Blunkett said, "and the system then spewed it out."

"And the system in the end did fast-track that particular application along with many others. And an e-mail was sent back which we were not aware of ... which actually said, 'no favors, but slightly quicker.' And once I had found that out yesterday, I realized that I had to resign," Blunkett said.

He described the anguish of recent weeks as the "worst of my life," but indicated he would continue his legal fight over the boy — and take comfort from his future relationship with him.

"The disappointment, and sometimes I think probably, the depression of the months to come are absolutely nothing compared with the joy of being able in the future ... to grow and work alongside — along with my older sons — my little lad," he said.

Earlier Wednesday, opposition Conservative leader Michael Howard seized on Blunkett's comments to his biographer about his colleagues.

Howard tossed the biography toward Blair during a debate in the House of Commons, suggesting that he read it so he could "give a full explanation of the government's total failure to deliver."

After Blair had left the chamber, a Labour Party member tossed the book back at the Conservative benches.

Blunkett's targets included Foreign Secretary Jack Straw, Treasury chief Gordon Brown and Education Secretary Charles Clark. And Blair, Blunkett told biographer Stephen Pollard, doesn't like people who stand up to him.

Blunkett showed signs of feeling the pressure — using a back door to leave his home Wednesday to avoid reporters, and canceling plans to appear in the House of Commons Wednesday afternoon.