LONDON – Britain will cut its forces in Iraq by half in the spring, shrinking the commitment of America’s leading military partner to just 2,500 troops engaged mainly in training Iraqi forces, Prime Minister Gordon Brown said Monday.

The announcement goes much further than a reduction of 1,000 troops the prime minister announced in Baghdad, the Iraqi capital, last week, and sets the stage for Britain’s exit as an active combat participant in the still-troubled region of southern Iraq where its troops are based.

U.S. officials said the move was consistent with plans Britain 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.

But privately, some U.S. officials complain that the new prime minister is abdicating his country’s role in the war, which is deeply unpopular in Britain, because it is political 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 his predecessor as prime minister, Tony Blair, whom he replaced in June. Brown called for Britain to move progressively out of active combat into a staged “overwatch” role in Iraq, with only “limited” capability for “reintervention” after spring.

The British contingent remains the largest of the foreign forces allied with the U.S. military in Iraq, but the overall number has dropped from about 50,000 in 2003 to fewer than 12,000 now. U.S. troops make up 93 percent of the foreign 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. A YouGov poll earlier this year showed that 30 percent of respondents wanted troops out as soon as possible, while another 40 percent wanted a time limit of no more than 18 months.

Thousands of protesters marched through central London to the P arliament Monday to voice opposition to a war in which 170 British soldiers have lost their lives.

(Begin optional trim)

Protesters were dismissive of the reductions Brown announced.

“The smaller the number of British troops is, the more stupid the British policy is. What can you do with two and a half thousand troops? It’s simply a political gesture to support George Bush,” said David Wilson, a spokesman for the Stop-the-War Coalition, which organized the march.

Similar sentiments were expressed inside the House of Commons.

“The hard truth is that Britain’s involvement in Iraq has been a catastrophe,” declared Liberal Democratic Party leader Menzies Campbell.

“We have paid dearly and widely in resources and reputation, and isn’t it time to acknowledge that the presence of British troops in Iraq no longer serves any realistic military or political purpose?” he said. “Isn’t it time to acknowledge that after four and a half years, Britain has more than fulfilled any moral obligation to the people of Iraq, and that our obligation now is to the young men and young women of our armed forces?”

(End optional trim)

Analysts said the force reduction, while it falls short of a full withdrawal, does signal 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 influential 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 the biggest reason for his downfall as prime minister. Blair also had called for gradually phasing down British troops as Iraqis took over responsibility, but showed more willingness to keep remaining forces 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.”

(Begin optional trim)

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 all been relinquished.

Brown said the plan now calls for going down to 4,000 troops and then 2,500 in the spring, “with a further decision about the next phase made then.” British analysts said it was clear Brown hopes to have all forces out of Iraq before elections tentatively expected in the spring of 2008.

At least 16 nations that originally sent troops have pulled out, including Japan, Spain, Italy and the Netherlands. A total of 25 nations, in addition to the U.S., remain part of the force, although not all are deploying troops.

In Washington, Gordon D. Johndroe, spokesman for the National Security Council, called Brown’s announcement “consistent with previously announced plans by the British to reduce their troop presence in southern Iraq as the Iraqi Security Forces are able to take lead responsibility in the southern provinces.”

“Moving to overwatch status is the desired outcome for all coalition forces in Iraq as the Iraqis continue to takeover more security,” he said.

Some U.S. officials have objected in private about Brown taking steps to speed up the British drawdown almost immediately after assuming office, even while Basra is beset by factional fighting.

In approving of earlier British drawdown plans, however, Lt. Gen. Raymond T. Odierno, the U.S. ground commander, said that the violence in Basra is rooted in an internal Shiite power struggle that he said the Iraqi government must resolve.

Likewise, Gen. David H. 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.”

(End optional trim)

Times staff writers James Gerstenzang, Julian E. Barnes and Peter Spiegel in Washington contributed to this report.

AP-NY-10-08-07 2059EDT