British Foreign Secretary Jack Straw said on Monday UK troops would remain to prevent a "security vacuum".
Speaking on BBC radio a day after a surprise visit to Iraq by Prime Minister Tony Blair, Straw said he could not give an "exact timescale" for the withdrawal of British forces.
"I can't say whether it is going to be 2006, 2007," he said, adding that in any event, "it is not going to be months".
Speaking to several hundred British troops in the southern city of Basra on Sunday, Blair did not say how long they would have to remain in the country they invaded alongside the United States nearly 10 months ago.
But the Guardian newspaper said he "made plain in private discussions with military commanders" that the number of British troops in Iraq was unlikely to drop "until well into 2005".
The Daily Telegraph said "several thousand British troops" would remain in Iraq at least until 2006.
Britain now has 10,000 soldiers in Iraq, compared with the 40,000 who participated in the 20 March invasion alongside the US that led to the overthrow of Saddam Hussein's regime.
UK has about 10,000 soldiers in
the oil-rich, fertile south
Straw said British forces - which occupy the oil-rich south of Iraq and the area's Gulf ports - were playing a vital role in ensuring stability and security in Iraq, as they had in Afghanistan.
Though US forces to the north still come under almost daily attack, the British sector has been relatively stable, and there have been no British combat fatalities since 31 October.
The southern sector is populated largely by Shia Muslims, who suffered under Saddam's Sunni-dominated rule. Shia clerics have largely urged their community not to fight the US-led occupation in the hope of an early transition to democratic independence.
On his flight back to London on Sunday, Blair told reporters the next six months were "very critical" in turning Iraq into a stable keystone of democracy in the Middle East.
"We have got to get on top of the security situation properly and we have got to manage the transition," he said. "Both of those things are going to be difficult."
Speaking on New Year's Day, Defence Secretary Geoff Hoon told the BBC British forces would be in Iraq in a year's time to support the incoming transitional government.
"I don't want to speculate precisely about numbers. I would expect to see the role of British forces change, subtly perhaps," he said.