LONDON — LONDON -- Britain will cut its force in Iraq by half in the spring, shrinking the commitment of the United States' leading military partner to 2,500 troops, most of them engaged mainly in training Iraqi forces, Prime Minister Gordon Brown said yesterday.
The announcement goes much further than a reduction of 1,000 troops that the prime minister announced last week in Baghdad, the Iraqi capital, and it sets the stage for Britain's departure as an active combat participant in the troubled region of southern Iraq, where its troops are based.
U.S. officials said the move is consistent with plans Britain had previously announced to reduce the size of a force that once numbered 40,000 soldiers. U.S. generals have said publicly that there is little the British can do to resolve the main conflict in the south, an internal power struggle among Shiite factions.
Privately, however, some U.S. officials say the new prime minister is reducing his country's role in the war, which is deeply unpopular in Britain, because it is politically expedient.
"We will continue to be actively engaged in Iraq's political and economic development. We will continue to assist the Iraqi government and its security forces to help build their capabilities - military, civilian and economic - so that they can take full responsibility for the security of their own country," Brown told the House of Commons.
The strategy he laid out was a departure from that of Tony Blair, whom he replaced as prime minister in June. Brown called for Britain to move away from combat into an "overwatch" role in Iraq, with limited capability for "re-intervention" after the spring.
The British contingent remains the largest of the coalition forces allied with the U.S. military in Iraq, but the number of British troops has dropped from about 50,000 in 2003 to fewer than 12,000.
U.S. troops make up 93 percent of the coalition force. While U.S. forces have been bogged down in Baghdad and other conflict-ridden regions to the north, they have relied on British forces to guard southern Iraq, a region that includes some of the nation's biggest oil fields, its only access to the sea, 200 miles of its long border with Iran and the main supply line from Kuwait.
Brown's government faces increasingly vociferous opposition to the war at home. In a YouGov poll this year, 30 percent of respondents said they wanted British troops out as soon as possible, and 40 percent said they wanted a withdrawal within 18 months.
Thousands of protesters marched through central London to Parliament yesterday to oppose a war in which 170 British soldiers have lost their lives.
Analysts said the force reduction signals Britain's unwillingness to accede to quiet U.S. requests for substantial help in patrolling the troublesome Iranian border.
"The mission instead is now to extricate the force responsibly and without damaging the area's precarious security, such as it is," Michael Clarke, director of the Royal United Services Institute, wrote on the institute's Web site.
"Exactly when Britain leaves Iraq has become a tough political question, but the drawdown and progressive disengagement, probably via basing in Kuwait, is effectively non-negotiable."
Blair's support for the war was his biggest political liability as prime minister. Blair also had called for gradually phasing out the British military presence as Iraqis took over responsibility, but he showed more willingness than Brown has to keep the remaining troops actively engaged in patrols and military operations, analysts said.
"With Brown, you can say there's an exit strategy," said Alex Bigham, a Middle East expert at the London-based Foreign Policy Center, which participated in a major study of Iraq this year. "Whereas under Tony Blair you still had British troops going out on the streets of Basra, that won't happen under Gordon Brown unless there's a very serious disturbance."
The core of Britain's remaining contingent redeployed several weeks ago from the center of Basra to the airport. Troops will remain at the airport, a decision that reflects its value as "a very key strategic asset in terms of supporting the troops who remain, supporting the Iraqis, and if necessary, bringing in forces from outside if things go bad," said Christopher Langton, a conflict analyst at the International Institute for Strategic Studies.
Britain has been progressively handing back control of large swaths of southern Iraq to newly trained Iraqi forces. Muthanna, Dhi Qar and Maysan provinces have been relinquished.
Brown said the plan calls for reducing the British force first to 4,000 troops, then to 2,500 in the spring, "with a further decision about the next phase made then."
British analysts said it is clear that Brown hopes to have all British forces out of Iraq before elections expected in the spring.
At least 16 nations that originally sent troops have pulled out, including Japan, Spain, Italy and the Netherlands. Twenty-five nations besides the United States remain part of the coalition, but not all are deploying troops.
In Washington, Gordon D. Johndroe, a spokesman for the National Security Council, called Brown's announcement "consistent with previously announced plans" by the British to reduce the number of their troops as the Iraqi security forces take over.
Gen. David Petraeus, the overall U.S. commander in Iraq, said during a visit to London last month that Iraqi security forces could handle any violence in the south "with minimal assistance."
Kim Murphy writes for the Los Angeles Times.