LONDON — Prime Minister Tony Blair, President Bush's closest ally, was likely to announce plans today to resign in the next year, forced by intense pressure within his Labour Party after nearly a decade in power.

Blair's official spokesman said a statement was "highly likely" during the prime minister's visit to a school in London in the early afternoon. The spokesman said Blair was not expected to set a precise date for when he would step down.

Eight junior officials quit Wednesday to insist on his resignation, and news reports said Blair and Treasury chief Gordon Brown, who is likely to be the next prime minister, had arguments in Blair's office about a handover date. Pressure for Blair to leave office or announce a departure date has intensified, fueled by anger at his handling of fighting in the Middle East and anxiety over Labour's slide in the polls.