LONDON – Britain’s top law enforcement officer, a key member of Tony Blair’s Cabinet, resigned yesterday after acknowledging that his department fast-tracked a visa for his former lover’s nanny.

David Blunkett’s departure was a blow for Mr. Blair, who had strongly backed him and confidently asserted he would be cleared of wrongdoing. Mr. Blunkett was in charge of key legislation on crime and terrorism, and was establishing a new system of national identity cards.

Mr. 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.

“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 Mr. Blunkett, who has gone to court to establish that he fathered Ms. 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.

As Britain’s only blind legislator, the divorced Mr. Blunkett gained some sympathy for his shattered love affair. His three-year relationship with the married Ms. Quinn soured over the summer, and Mr. Blunkett has gone to court in an attempt to gain access to the 2-year-old son.

Ms. Quinn, currently expecting another child whose paternity is the subject of intense speculation in the press, denies the toddler is Mr. Blunkett’s son.

Mr. Blunkett has suggested Ms. Quinn leaked details of their relationship and the “Nannygate” claim to the press as a form of revenge.

Mr. 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.

Mr. Blair yesterday appointed the education secretary, Charles Clarke, as Home Secretary to replace Mr. Blunkett. Mr. 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 counter-terrorism.

Mr. Blunkett’s problems have been a distraction and embarrassment for Mr. 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.