It was the second time in less than a year that Blunkett, who was works and pensions secretary, has quit Blair's cabinet, after being forced to resign last December as home secretary.

Blair's official spokesman said the prime minister accepted the resignation "reluctantly," after Blunkett, 58, told him he felt his position had become "untenable".

The prime minister was to face parliament at noon (1200 GMT) for a weekly question period certain to be dominated by the resignation.

Political analysts said Blunkett's downfall was a serious blow to Blair, just six months into what the prime minister wanted to be a triumphant third straight term in power for his Labour Party.

The sudden development came just a day after Blunkett gave an interview to his hometown newspaper in northern England, the Sheffield Star, declaring that he would not allow his critics to drive him out of office.