LONDON – Prime Minister Tony Blair, the most successful politician in the Labor Party’s history, has announced that he will resign on June 27.

The announcement, anticipated for months, triggers a party leadership contest that is expected to confirm Gordon Brown, the chancellor of the exchequer, as Blair’s successor.

Blair briefed his Cabinet on the timing of his departure Thursday morning before flying up to his Sedgefield district in northeast England to outline his decision in a speech to his local constituents and the nation. “I’ve been prime minister of this country for just over 10 years,” he said.

“In this job, in the world of today, I think that’s long enough for me but more especially for the country. Sometimes the only way you conquer the pull of power is to set it down.”

Blair marked his 10th anniversary in office earlier this month. He is the Labor Party’s longest serving prime minister, and the first to win three successive elections.

But the Iraq war and his alliance with President Bush, a widening scandal over party finances and the simple fact that a decade is a lifetime in politics cut into his popularity, and Labor has trailed the opposition Conservative party in the polls for about a year.

After the 2005 election, Blair declared his intention of serving a full third term, but that became untenable as his poll ratings continued to sag.

He became convinced it was better to avoid the fate of the longest serving prime minister in modern Britain, Margaret Thatcher, and jump before he was pushed.

“I came into office with high hopes for Britain’s future, and I leave it with even higher hopes for Britain’s future,” Blair told his supporters.

“I give my thanks to you, the British people, for the times that I have succeeded – and my apologies for the times I have fallen short,” he said.

Brown, who was Blair’s partner in the revival of the Labor Party’s fortunes in the early 1990s and later his chief rival for its leadership, has already declared his candidacy to replace Blair and is expected to face little competition from within the party.

When Blair moves out of Number 10 Downing Street on June 27, Brown would then move in. He will most likely not have to face the voters until 2009.

Blair leaves behind a legacy of accomplishment that includes a peaceful Northern Ireland, years of steady economic growth, and a Labor Party that has become a viable alternative for Britain’s increasingly prosperous middle class.

“There are obviously judgments to be made on my premiership and in the end that is for you the people to make,” Blair told his Sedgefield constituents. He will remain a Member of Parliament from the district until the next election.