LONDON – Tony Blair will leave office on July 26, the Sun newspaper reported in its Wednesday edition, as the increasingly unpopular prime minister faced growing pressure to quit from within his own Labor Party.

The report comes a day after a top Blair ally said the prime minister would probably leave office within a year.

The Sun said Blair would step down as head of the Labor Party on May 31, less than a month after his tenth anniversary in office. He would resign as prime minister eight weeks later, after an election to choose a successor as party leader, expected to be his finance minister, Gordon Brown.

Blair, winner of a record three consecutive elections for Labor, has already said he will not stand for a fourth term.

His popularity has plunged after a series of government scandals over sleaze and mismanagement, as well as controversy over the Iraq war. Opinion polls put Labor well behind the opposition Conservatives — resurgent under their youthful, pro-environment leader David Cameron.

Blair’s office declined to comment on the Sun story: “We have no intention of commenting on any speculation of the timetable,” a spokeswoman said.

The right-leaning tabloid, Britain’s largest-selling daily newspaper, has a reputation for accurate political scoops.

“It’s been known to me for some time that the prime minister has had a date in mind, and we’ve been working hard on it,” political editor George Pascoe-Watson told Sky News television.

“We’ve actually been able to nail down the date that he has put in his diary.”

Blair has said he would leave enough time for a transition to his successor, but has angered many in his party by refusing to name the date he would leave.

The speculation reached a fever pitch over the past 24 hours after newspapers reported that once-loyal Labor members of parliament had signed a letter calling on Blair to step aside.

Environment Minister David Milliband, a loyal Blair ally, responded on Tuesday by suggesting he would be gone within a year, remarks taken as authorized by Blair.

Blair, 53, won his first term on May 1, 1997. A decade in power would leave him a year short of Margaret Thatcher’s record as the longest-serving prime minister in more than a century.

Pascoe-Watson said he believed Brown was aware of the date planned for the transfer and was keen that Blair should make it public.

“The interesting question is whether or not the chancellor knew privately of the date, and I believe the chancellor did know of the prime minister’s intentions to leave during this timetable. I think the chancellor would like the prime minister to say this in his own words.”