LONDON - Gordon Brown, all but certain of becoming Britain's prime minister at the end of next month, launched his formal campaign for the job yesterday by acknowledging that mistakes had been made in Iraq but promising that Britain would stand by its obligations to the Iraqi people.

Brown received a strong endorsement from Prime Minister Tony Blair, who announced Thursday that he would step down June 27.

"He has a quite extraordinary and rare ability, a tremendous talent to be put at the service of our country," Blair said of the man who has been waiting - not always patiently - to take the job Blair has held for the last decade.

Brown, the chancellor of the Exchequer, faces no serious challenge in his bid to lead the Labor Party and the keys to No. 10 Downing Street.

The few serious contenders from the Blair cabinet who considered a challenge have withdrawn. Two little-known members of Parliament from the party's left wing have said they will mount a symbolic challenge.

With a new "Gordon Brown for Britain" logo as a backdrop, Brown opened his campaign with a televised address in the capital by promising to "listen and learn" about people's concerns.

"I want to lead a government humble enough to know its place, where I will always strive to be - and that is on the people's side," he said.

"Government must be more open and more accountable to Parliament - for example in decisions about peace and war, in public appointments and in a new ministerial code of conduct."

On Iraq, Brown said: "I accept that mistakes have been made."

Britain is the Bush administration's most important ally in Iraq, and Blair has been an articulate proponent of the global war on terrorism even though it has hurt his standing in the polls. No significant change in Britain's policies is expected under Brown.

"We will keep our obligations to the Iraqi people. These are obligations that are part of a U.N. resolution; they are in support of a democracy," Brown said.

"I do think that over the next few months the emphasis will shift," he said. "We've got to concentrate more on political reconciliation in Iraq. We've got to concentrate more on economic development so that people in Iraq . . . feel that they have a stake in the country for the future."