The number of UK troops in Iraq is set to be "significantly lower by a matter of thousands" at the end of next year, the defence secretary has said.
A "drawing down" of troops did not mean a withdrawal, he said
Des Browne said it was hoped that local Iraqi forces would take control of Basra, in the south of the country, in the spring.
But a "drawing down" of troops did not mean a withdrawal because the Iraqis would still need back-up, he said.
The UK has about 7,000 troops in the south of Iraq, mostly around Basra.
Mr Browne restated Foreign Secretary Margaret Beckett's assertion that British-controlled Maysan province could transfer to Iraqi control in January, followed by Basra shortly afterwards.
But a handover did not mean a withdrawal of troops, he said.
"Even when all of the provinces are handed over, we will still be providing a force to mentor and back up the Iraqi army and police and to protect coalition supply routes," he said.
"I do not believe it is right to give precise numbers, nor to assume what the next 12 months will hold.
"But 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."
He would not allow "a single one of the 7,000 total British soldiers, sailors and air personnel to stay in Iraq longer than necessary", he said.
He also warned that insurgent attacks could rise in the coming weeks and months.
"As we move towards handover, perversely, the number of attacks on us may increase," he said.
Mr Browne said lasting progress in Iraq could only be achieved with "security, politics and economy" with security having priority.
And he called on Iraq's neighbours to play their part in helping its reconstruction, saying the behaviour of Iran remained "a deep concern".
"It has influence inside Iraq, the power to turn up or down the heat, to turn on or off the dialogue," he said.
"It is not using that influence well."
The message to Iran was simple, he said.
"Be a constructive partner. Help yourself - as well as the wider region - or face increasing isolation."
Conservative defence spokesman Liam Fox questioned Mr Browne's decision to "put timelines on all of this".
"It may well be affected by events which leads to disillusionment back at home," he said.
"Also, setting a timeline is a green light for insurgents to try to disrupt that timetable."
Lib Dem defence spokesman Nick Harvey, meanwhile, said "vague reassurances" were not enough.
"The government must urgently lay out its plan for a phased withdrawal of troops and the transfer of security to the Iraqi security forces," he added.
Last week, announcing plans to hand over the remaining British-controlled provinces to the Iraqis, Mrs Beckett said leaving too early would make the bloodshed "even worse".
A UN report has said the Iraqi death toll hit a record high in October, with more than 3,700 people killed in the continuing violence.