UK to withdraw 3,800 troops from Afghanistan during 2013
David Cameron has told MPs that 3,800 British troops - almost half of the current force serving in Helmand province - are to be withdrawn from Afghanistan next year.
Troop numbers are already being reduced from 9,500 to 9,000 before Christmas.
And numbers would fall to about 5,200 by the end of 2013, Mr Cameron told MPs at Prime Minister's Questions.
All Nato operations are due to finish by the end of 2014, with responsibility being transferred to Afghan forces.
But a small number of British troops would remain in the country beyond that, working at an officer training academy and "involved in returning equipment and dealing with logistics", Mr Cameron said.
Mr Cameron told MPs "we'll be able to see troops come home in two relatively even steps", in 2013 and 2014.
This was due to "the success of our forces and the Afghan national security forces, and the fact that moving from mentoring at a battalion level to mentoring at a brigade level in 2013", he said.
In a subsequent Commons statement, Defence Secretary Philip Hammond added: "Over the last 11 years we have been helping to ensure that Afghanistan's past is not inevitably its future.
"As we move towards full transition at the end of 2014, it is clear that there remain huge challenges ahead for the Afghan people.
"Our combat mission is drawing to a close, but our commitment to them is long term."
He told MPs that the UK had "on-going funding commitments" to the country amounting to £250m a year.
Mr Hammond suggested that, if this level of aid spending were to continue, "the Afghan government will need to address the corruption which remains rampant, and could become a very real threat to the long-term stability of Afghanistan".
But he praised democratic reforms in the country, which meant "Afghan voters can look forward to a future of their choosing, rather than one forced upon them".
Labour leader Ed Miliband said that "greater diplomatic efforts" would "give us our best chance of leaving behind an inclusive and durable settlement in Afghanistan".
Responding to Mr Hammond's statement, shadow defence secretary Jim Murphy added: "This is the fourth conflict in Afghanistan and we have no intention of there ever being a fifth."
Conservative MP and chairman of the defence select committee James Arbuthnot noted that troop withdrawals would leave those remaining in the country increasingly isolated.
Former Lib Dem leader Sir Menzies Campbell also drew attention to the vulnerability of military equipment remaining in the country that "might be of use to insurgents or others of malign intention to the government of Afghanistan".
Labour backbencher David Winnick concluded that "military victory of any kind against the Taliban is totally out of the question: it hasn't come about so far; it's not going to come about in the next two years".
Earlier, a spokesperson for Afghan President Hamid Karzai confirmed he had spoken to the Mr Cameron by phone.
A statement from the Afghan presidential palace said: "They talked to each other about the peace process, the successful security transition and about the agenda of trilateral talks between Afghanistan, Pakistan and UK which are going to be held next month."
The decision on the pace of withdrawal was agreed at a meeting of the National Security Council on Tuesday, at which officials said politicians and military chiefs were in consensus on the timetable.
Mr Cameron also discussed Afghanistan in an hour-long video conference with US President Barack Obama, during which they agreed that Nato's strategy to withdraw combat troops by the end of next year was "on track".
"This would present further opportunities for International Security Assistance Force countries to bring troops home next year and they agreed to stay in close touch as detailed plans develop," a Downing Street spokesman added.
"They also agreed on joint work to strengthen the political process, particularly supporting Afghanistan and her neighbours to work together for stability, building on the trilateral discussions with Pakistan led by the UK."
The US currently has about 60,000 troops in Afghanistan, after withdrawing some 23,000 this year. The White House is expected to spell out its plans for withdrawal in January.
Mr Cameron said he was "confident" of meeting his promise to bring the majority of UK troops home by the end of 2014 while on a trip to Afghanistan in July.
He said reductions in troops before then would be done in an "ordered and sensible" fashion.
Since 2001, 438 British personnel have died in Afghanistan.