Britain said on Monday it expected to withdraw thousands of troops from Iraq by the end of next year, in the clearest pledge yet of a pullout from the increasingly violence-scarred country.
In a key policy speech in London, Defence Secretary Des Browne also urged Iran to stop its "unacceptable" behaviour in fuelling attacks there and become partners with the West, or face increasing isolation.
Britain currently has some 7,100 troops in southern Iraq, three years after supporting the 2003 US-led invasion, but like the US administration has faced growing public pressure to withdraw its forces.
"I can tell you that by the end of next year, I expect numbers of British forces in Iraq to be significantly lower, by a matter of thousands," Browne told the Chatham House think tank in London.
He said military planners had been studying how to reduce the troop presence for months.
"In the end of course, it must depend on conditions on the ground, including the level of threat and the capacities of Iraqis to deal with it, and the final decision will be down to our commanders," he said.
He outlined an accelerated strategy in which British troops train Iraqi forces, gradually transfer security responsibilities to them, and withdraw to the background while keeping rapid response forces ready if needed.
Admitting security was improving faster in some parts than others, he recalled that the army transferred control to Iraqi forces over the summer in Al-Muthanna and Dhi Qar, two of the four provinces in the British sector.