BAGHDAD (AP) -- Britain's prime minister announced Wednesday that his country's soldiers will leave Iraq by the end of May, a move that will force the U.S. to send troops into the Shiite south to secure supply lines to the much larger American force.
Britain, with some 4,000 troops currently in Iraq, is the second-largest contributor to the international military coalition after the United States.
But the war has been extremely unpopular in Britain, and the end of the mission could be a political boon to Brown if he calls, as expected, a national election for June 4.
Britain had previously said the mission would conclude in the early summer, and Brown's statement appeared to accelerate that timetable.
Although security in Iraq has improved markedly in recent months, a double bombing in Baghdad during Brown's visit highlighted the violent passions that persist.
Police said 18 people were killed and 52 wounded when a car bomb exploded in eastern Baghdad and a roadside bomb went off minutes later as police rushed to the scene. The U.S. military reported nine killed and 43 wounded; conflicting casualty tolls are common in Iraqi bombings.
Late Wednesday, a bomb exploded moments after a U.S. military patrol passed by in the mostly Sunni Baghdad neighborhood of Sulaikh, killing a 13-year-old boy and wounding three other people, an Iraqi military officer at the scene said on condition of anonymity.
And the U.S. military said Iraqi forces killed three assailants who opened fire on them northwest of Baghdad and detained four suspected insurgents.
"We have agreed today that the mission will end no later than the 31st of May," Brown said at a news conference with Prime Minister Nouri al-Maliki.
Al-Maliki thanked the British for their efforts in "getting rid of dictatorship and terrorism."
"They have made a lot of sacrifices," al-Maliki said.
More than 45,000 British troops took part in the 2003 U.S.-led invasion that toppled Saddam Hussein - compared with about 250,000 American troops in the invading force. At least 173 British soldiers have died in Iraq, compared with more than 4,200 Americans.
Britain's decision means the United States will have to send forces to the south to help with security and protect vital supply routes from Kuwait to the nearly 150,000-strong American force in central, northern and western Iraq.
A U.S. brigade is expected to take over Britain's base next to Basra's international airport once British forces leave.
The new mission will pose more challenges to the U.S. military as it moves to end its presence in Iraq's cities by the end of June and from all of Iraq by 2012 under terms of a new security pact.
Most of the area between Basra and Baghdad already has been put under the control of the slowly reviving Iraqi military.
Brown went to Basra after meeting with al-Maliki and visited the nearby port of Umm Qasr, Iraq's only sea outlet. He then left for Britain.
He said the region has seen significant security improvements and that "this reflects the cooperation between local security forces and local authorities with the U.K. troops."
Before leaving Baghdad, Brown also said that he and al-Maliki discussed the fate of five Britons who were kidnapped by a Shiite militia a year and a half ago.
Little information 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.
Brown emphasized that despite the military withdrawal, Britain will remain strongly engaged in Iraq.
"We will take forward a collaborative approach to build skills and capacity for the government of Iraq as a source of crucial revenue - a priority will be developing the expertise of the Oil Ministry," he said. "And we are in discussion with many of our own companies who are interested in investing in Iraq."