David Blunkett, a key ally of Prime Minister Tony Blair, resigned from the cabinet Wednesday for the second time in a year after he admitted violating ethical guidelines. Critics described his departure as further evidence of Blair's waning authority.

"I'm guilty of a mistake and I'm paying the price of it," Blunkett said at a news conference, adding that he was "deeply sorry" for embarrassing Blair. The prime minister had reappointed Blunkett to the cabinet in May after he stepped down last December because of a different scandal, which involved using his office to expedite a visa application for his lover's nanny.

Blunkett, one of the most intriguing figures in British politics, overcame great hardship to rise to power. He has been blind since birth and grew up in a blue-collar family that endured the tragic loss of his father, a gas company worker, when he fell into a vat of boiling water.

Blunkett was first appointed education secretary in 1997, then served as the powerful home secretary and most recently as the minister overseeing work and pensions.

In Parliament on Wednesday, Blair praised Blunkett, saying, "I've always believed, and believe now, that he is a decent and honorable man, who has contributed a great deal to his country, who has overcome immense challenges that frankly would have daunted the rest of us."

Blunkett's current trouble involves failing to get approval from a watchdog committee before taking a post with and investing in a technology firm, DNA Bioscience, last April, while he was briefly out of government. Recent as well as current members of the government must disclose their business interests.

Blunkett's public problems began last year with disclosures about his affair with Kimberly Quinn, the American-born publisher of the weekly Spectator magazine. Blunkett, 58, had long been divorced, but Quinn was married. After the affair ended, Blunkett and Quinn had a bitter feud, and Blunkett successfully proved that she had given birth to his child. But Blunkett's efforts to obtain the visa for the nanny cost him his job as home secretary.

Since then, Blunkett's moves and missteps have been chronicled widely in the media. A sold-out stage play and a much-hyped TV show both depicted Blunkett as a farcical, cartoon-like character.

Blunkett has been a strong supporter of Blair even as other members of the Labor Party have shifted allegiance to his likely successor, Gordon Brown, chancellor of the exchequer, and increasingly votes against the prime minister's proposals.

Members of the opposition Conservative Party, including leader Michael Howard, said the departure was the latest blow for Blair, who Howard said was "hemorrhaging" his authority. "For how long will this country have to put up with this lame-duck prime minister, in office but not in power?" Howard said in Parliament.

David Butler, an Oxford professor and author of many books on British politics, said Blunkett, though an effective minister, had become a liability for Blair and had done the right thing by resigning. Blair "did stumble a bit on this, and it doesn't do him any good," Butler said, "but it certainly is not life-threatening to his term in office."

David Blunkett leaves the House of Commons after resigning as work and pensions minister because of an ethics breach while out of office.