Around 3,800 British troops will be withdrawn from Afghanistan during 2013, David Cameron has announced.
The Prime Minister told the House of Commons that the strength of the UK force will be reduced from 9,000 to 5,200 by the end of next year.
The partial withdrawal paves the way for the final removal of the bulk of British personnel from the central Asian country as planned by the end of 2014, said Mr Cameron.
Mr Cameron's announcement during the weekly session of Prime Minister's Questions came around half an hour ahead of the quarterly update on the situation in Afghanistan to MPs by Defence Secretary Philip Hammond, who was expected to give further details of the drawdown plans.
The Prime Minister told MPs: "We have two decisions to make. First of all, the decision about the drawdown of troops between now and the end of 2014.
"What the Defence Secretary will announce is that because of the success of our forces and the Afghan National Security Forces, and the fact that we are moving from mentoring at a battalion level to mentoring at a brigade level by the end of 2013, we will be able to see troops come home in two relatively even steps - 2013 and 2014 - leaving probably around 5,200 troops after the end of 2013, compared with the 9,000 that we have now.
"In terms of post-2014, we haven't made final decisions yet. We've said very clearly - no-one in a combat role, nothing like the number of troops there are now. We've promised the Afghans that we will provide this officer training academy that they've specifically asked for. We are prepared to look at other issues above and beyond that, but that is the starting baseline."
Labour leader Ed Miliband asked: "Given that thousands of troops are still going to be in harm's way in Afghanistan, can the Prime Minister say what specific effort the Government is making with the international community to match the continuing military effort with the greater diplomatic efforts that I know he and I both think are important? After all, this is our best chance of leaving behind an inclusive and durable political settlement in Afghanistan."
Mr Cameron said Mr Miliband was "entirely right that as well as a military track there has always been a political and diplomatic track", adding: "After December 2014, there will still be some troops involved in returning equipment and dealing with logistics. Exact announcements will be made about that at a later stage. We will not be leaving Afghanistan in terms of our support and our help for the Afghans. We will be contributing £70 million a year to help pay for the Afghan National Security Forces, we will have an aid programme in excess of £70 million a year for Afghanistan.
"I hope to host further meetings including early in the new year and I spoke to President (Hamid) Karzai this morning to encourage him to keep working on this vital relationship so that Pakistan and Afghanistan can both see they have a shared interest in a stable future."