Around 3,800 British troops will be withdrawn from Afghanistan during 2013, David Cameron has announced.
The prime minister told the House of Commons in Wednesday 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.
"It's a good moment again to pay tribute to the incredible work they've done, many of them going back for tour after tour. The ones I've spoken to recently have been particularly impressed by the capacity of the Afghan national forces.
"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, which is so important."
Mr Cameron said Mr Miliband was "entirely right that as well as a military track there has always been a political and diplomatic track".
The PM added: "Let me be clear - 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.
"In terms of the diplomatic track, the thing we are most focused on is bringing Afghanistan and Pakistan together, and I have personally hosted two meetings between the two presidents.
"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."
Mr Hammond later told MPs that the UK was able to reduce force numbers in Afghanistan because of the "rapidly improving" capability of the home-grown Afghan National Security Forces, which are now expected to take over security responsibility for the whole of the country in the middle of 2013.
And he said there had been a reduction in Taliban attacks which indicated that the insurgency was now "weakened".
The drawdown of UK personnel was in line with the policy of the Nato-led International Security Assistance Force, said Mr Hammond. Britain and Nato "fully support" the ambition of the Kabul government to take on responsibility for security across the whole of Afghanistan by the end of 2014
The defence secretary told MPs: "In accordance with Isaf planning, by the end of 2013 we expect that UK forces will no longer need to routinely mentor the Afghan National Army below brigade level. This is a move up from our current battalion-level mentoring and is a reflection of rapidly-improving Afghan capacity and capability.
"A progressive move to brigade-level monitoring will also allow us to make further reductions to our force levels from the 9,000 that we will have at the end of this year.
"Our current planning envisages a reduction to around 5,200 by the end of next year. This number is based on current UK military advice and is in line with the Nato strategy agreed at Lisbon and the emerging Isaf planning. It also reflects the real progress being made in Helmand.
"We will keep this number under review as the Isaf plan firms up and other allies make drawdown decisions in the new year."
Mr Hammond told MPs the reduction was possible "because of the success of the Afghan National Security Forces in assuming a leading role".
The ANSF now have lead security responsibility in areas which are home to three-quarters of the country's population, and are providing a lead role on more than 80% of conventional operations and carrying out 90% of their own training, he said.
"They set their own priorities, lead their own planning and conduct and sustain their own operations. By the middle of next year, marking a moment of huge significance for the Afghan people, we expect the ANSF to have lead security responsibility for the whole country."
In Helmand province, where the bulk of UK troops are based, the ANSF are now "firmly in charge" in populated areas and are "showing an appetite to conduct Afghan intelligence-led raids".
Insurgents "remain committed to a campaign of violence in Afghanistan (and) continue to represent a threat to the future stability of the country", warned Mr Hammond.
But he added: "The ANSF, supported by Isaf where necessary, are taking the fight to the insurgents and pushing them away from towns, markets, key transport routes and intensively farmed areas towards the rural fringe.
"While we cannot be complacent, the picture as a whole is of an insurgency weakened. Enemy-initiated attacks have fallen by an average of over 10% in those areas that have entered the transitional process, demonstrating that the Afghans are managing their own security.
"More importantly, the geographical pattern of enemy-initiated attacks shows a significant reduction in impact on the local population."
Mr Hammond confirmed that Britain intends to end its combat mission in Afghanistan by the end of 2014, but said: "Our clear message to the Afghan people remains one of firm and ongoing commitment" on issues ranging from aid and diplomatic support to military training.
He urged the Afghan government to address "rampant" corruption.
But he insisted: "Democracy is taking hold in Afghanistan. Not, of course, in the same shape as here in Britain, but Afghan voters can look forward to a future of their choosing, rather than one that is imposed upon them. Afghan women enjoy a level of participation in their society and their politics that few could have dreamed of even half a decade ago."
The best opportunity for a stable and secure Afghanistan for the long term lies in a political settlement, which will require the Afghan government, the Taliban and other Afghan groups to come together to talk and to compromise, said Mr Hammond.
"Our aim is to generate confidence and dialogue. Our message to the Taliban is that reconciliation is not surrender; it is an opportunity for all Afghans to sit down together and help shape their country's future.
"Common ground can be found, focused on the need for a strong, independent, economically-viable Afghanistan."
He added: "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.
"Progress is clear and measurable and our determination to complete our mission and help Afghanistan secure its future remains undiminished."