LONDON -- Britain will halve its remaining troop contingent in Iraq next spring, Prime Minister Gordon Brown announced Monday. A British official later said they could not guarantee that any troops would remain in Iraq by the end of 2008.

Brown, under fire over his decision not to call an election for this year, said Britain would lower troop levels to 2,500 by mid-2008 and redeploy logistics staff to neighboring states. The British leader was clearly hoping the announcement would help boost his popularity among a public weary of the war.

Aides had stoked election rumors for weeks, particularly as lawmakers and activists gathered for a series of political party conferences. But Brown scrapped the plans Saturday as opinion polls suggested his early wave of public support had waned.

Brown told lawmakers Monday his Iraq plan follows the success of the U.S. troop increase this summer and efforts by Iraqis to drive suspected al-Qaida militants from havens in Anbar province, west of Baghdad.

He said decisions on further cuts would be made once the reduction to 2,500 was complete, rejecting a call from opposition lawmakers to set a timetable to withdraw all British forces.

Officials said the latest troop cut would be complete by April, and that a total withdrawal of forces would be among options considered then.

"At the point where we arrive at that number next year, we shall have a much clearer idea of what our policy is going to be," a British official said, speaking on condition of anonymity because of the sensitivity of the issue. "But certainly at this stage there's no guarantee they're going to be there beyond the end of (2008)."

The British presence in Iraq peaked with 46,000 troops during the March 2003 invasion. It was reduced to 18,000 that May, and 8,600 by the end of May 2004. This past May, there were about 5,500 British troops in Iraq.

Britain is already scaling back forces, and by the year's end will have 4,500 troops based mainly on the fringe of the southern city of Basra, where a power vacuum has exacerbated discord among rival Shiite groups.

Iraqi forces will take control of security in the southern province of Basra within two months, ending Britain's combat role in the country, Brown said.

Brown, who visited Iraq last week, said Monday that British forces will initially carry out oversight duties including securing key supply and transit routes from Kuwait to Baghdad. By next spring, troops will be focused mainly on training and mentoring.

Some 500 British logistics and support staff will be moved outside Iraq, but within the Middle East, to support the remaining troops, Brown said.

U.S. military officials are concerned that the reduced British presence in southern Iraq could open security gaps along routes to and from Kuwait.

The roadways are a lifeline for U.S. forces. And everything that the Americans can't fly out of the country when they eventually leave must make the potentially dangerous road journey to Kuwait through Basra province.

The American military is also concerned about the security of the southern oil fields and fear the absence of a major British force will discourage future investors deemed essential to upgrading Iraq's decrepit petroleum infrastructure. Security along the Iranian border should the British leave is another worry.

Nevertheless, the Bush administration said it welcomed the British announcement.

"Moving to overwatch status is the desired outcome for all coalition forces in Iraq as the Iraqis continue to take over more security," National Security Council spokesman Gordon Johndroe said.

Britain's participation in the 2003 U.S.-led invasion -- and the continuing presence of troops in Iraq -- remains deeply unpopular here. On Monday more than 2,000 people marched from London's Trafalgar Square to Parliament to demand a complete withdrawal of British troops.

Since the invasion, 170 British soldiers have died in Iraq.

Brown was apparently hoping his decision to pull troops from Iraq would distance him from his predecessor Tony Blair, whose parliamentary majority was reduced in 2005 by voters angered over the war.

He also is seeking to recapture the opinion poll lead he had soon after taking office in June. He lost the lead last week after the main opposition Conservatives unveiled popular tax cut pledges.

Analysts said the erratic polls demonstrate that Britain remains unsure whom it wants as leader since Blair departed in June.

Brown may also have handed his rivals a potentially crucial weapon by casting himself as a chronic procrastinator in failing to hold an autumn poll.

The ex-Treasury chief is remembered for his caution over whether to challenge Blair for their party's leadership in 1994, eventually standing aside for his longtime rival.

"I did consider holding an election. Yes, I looked at it," Brown told a Downing Street news conference. But he decided he wanted to put "my vision of what the future of the country was to the people of the country," before seeking their votes.

"The fact of the matter is I take responsibility. I take the blame. I take the decisions. If anything goes wrong, it comes back to me," Brown said.