BAGHDAD — British Prime Minister Gordon Brown said today his country, the second-largest military presence in Iraq after the United States, will end its mission in Iraq by May 31.

Britain had previously said the 4,000-strong mission would conclude in the early summer, and Brown's statement appeared to accelerate that timetable.

The announcement came on a violent day in Iraq, where police said a double-bombing in eastern Baghdad targeting traffic police left at least 18 people dead and 52 others wounded.

"We have agreed today that the mission will end no later than 31st May," Brown said at a news conference with Prime Minister Nouri al-Maliki.

The news conference took place without disruption, in contrast to one on Sunday with President George W. Bush, when an Iraqi journalist hurled both his shoes at Bush. The journalist, Muntadhar al-Zeidi, appeared before a judge Wednesday in his jail cell.

"We thank (British troops) for the efforts they have made for getting rid of dictatorship and terrorism. They have made a lot of sacrifices," al-Maliki said.

Brown also said he and al-Maliki discussed the fate of five Britons who were kidnapped by a Shiite militia a year and a half ago.

The men — information technology consultant Peter Moore and four guards — were seized from the Iraqi Finance Ministry compound in Baghdad in a brazen raid May 29, 2007.

Little information on them has come since, although a British newspaper reported this summer that the militia said one of the hostages had committed suicide.

"I call for all those who are holding them to let them return to their families immediately and without condition," Brown said.

Britain's withdrawal plans come amid a general improvement of security in Iraq. But severe problems remain, underlined by today's double bombing.

An Iraqi police official says the first blast this morning was from a car bomb. Moments later, a roadside bomb went off nearby, targeting people rushing to the scene.