LONDON – British troops in Iraq have an important job to do and there is no fixed timetable for their withdrawal, Prime Minister Brown said yesterday, despite speculation that Britain will soon announce a major pullout.

Mr. Brown has said a decision on the future of the 5,500 British troops in Iraq could be announced when Parliament returns from its summer break in October. But in a letter to Liberal Democrat leader Menzies Campbell, Mr. Brown rejected the opposition party’s demand to rethink the Iraq strategy.

“As I have made clear, decisions on U.K. force levels and posture in Iraq are dictated by conditions on the ground,” Mr. Brown wrote in the letter, which was released by his office.

Mr. Brown said Britain would “fulfill our obligations to the government and people of Iraq and the United Nations.” He rejected calls for “a predetermined exit timetable that would undermine our international obligations, as well as hindering the task of our armed forces and increasing the risks they face.”

British troops are based around the city of Basra, 340 miles southeast of Baghdad. Their number has been cut from 7,000 since the start of this year and is due to fall by a further 500 in the next few weeks when Britain pulls troops out of its last downtown base at Basra Palace, moving all its forces to the airport on the city’s outskirts.

Following the American-led invasion in 2003, Britain controlled security across southern Iraq, but has since handed over three of four provinces to Iraqi forces. Officials say Britain hopes to pass control of Basra, the remaining district, to local forces by the end of the year.

“Our aim is that Iraqi security forces will be capable of delivering security across the south and that we will be able to draw down our forces,” Mr. Brown wrote.

Enemies and allies alike have recently claimed British troops will soon leave Iraq in defeat.

Last week, former American Army General Jack Keane, who was vice chief of staff when the Iraq war began, said Britain had allowed security in southern Iraq to get worse.

He warned that American troops may need to plug the gaps if Mr. Brown withdraws significant numbers of British soldiers.

Also last week, Iraqi Shiite militia leader Muqtada al-Sadr was quoted as saying that British forces had been defeated in Iraq and would be forced to pull out sooner than they planned.

“The British have given up and they know they will be leaving Iraq soon,” Mr. al-Sadr was quoted as saying by The Independent newspaper.

British officials say an increase in militia attacks was expected as Britain prepares to hand over control of Basra to Iraqi forces. In all, 168 British personnel have died in Iraq since the invasion in March 2003.

Mr. Brown wrote that it was “wrong to say that the continuing presence of U.K. forces in Iraq will achieve little, or that they are severely restricted in what they can do.”

“U.K. forces in Basra continue to have the capability to strike against the militias and provide overall security,” he said.

“They will continue to work with the Iraqi authorities and security forces to get them to the point where they can assume full responsibility for security.”