UK to withdraw 3,800 British troops by the end of 2013, with government insisting Afghan security forces can lead in Helmand

David Cameron has confirmed that British troops will begin to return home from Afghanistan in their thousands next April as ministers urged the Taliban to engage in peace talks and accept that "reconciliation is not surrender".

Shortly after making the announcement, Philip Hammond, the defence secretary, conceded that the withdrawal of Nato forces would lead to "messy compromises", and that in all likelihood, "some parts of Afghanistan will not be under central government control".

"It is not a perfect democracy and it never will be," he said.

Hammond's stark assessment came as the government insisted that Afghan security forces were now strong enough to take the lead in combat operations in Helmand province, where British forces have been based since 2006.

Cameron said this allowed the UK to plan for the withdrawal of 3,800 British troops by the end of 2013, leaving just over 5,000 still in the country.

Though no such decisions have yet been taken, senior officers believe another 4,000 troops could return the following year, leaving a rump of 1,000 to provide logistical and training support for local forces, and to continue the monumental effort of bringing all the military's kit and equipment home.

During prime minister's questions, Cameron said he believed the conditions were now right for British troops to "come home in two relatively even steps in 2013 and 2014, leaving probably around 5,200 troops after the end of 2013".

Cameron said senior officers had told him they were impressed with the capabilities of Afghan forces, despite the setbacks of "green on blue" attacks this year in which 12 British troops have been killed by their Afghan colleagues.

"In terms of post-2014, no decisions have yet been taken, but we have said very clearly there will be nobody in a combat role and nothing like the number of troops now. We have promised the Afghans we will provide the officer training academy they have specifically asked for, and we are prepared to look at other issues above and beyond that, but that is the starting base line."

During defence questions, Hammond told MPs British plans would be under constant review from commanders on the ground, but that some troops would come home next spring, and another, larger, group would return in the autumn.

"We will keep this number under review as other allies make drawdown decisions in the new year. Let me be clear: this reduction is possible because of the success of the Afghan national security forces in assuming a lead role."

He claimed insurgent attacks in areas already under the control of Afghan forces had fallen on average by 10%, which demonstrated that "the Afghans are managing their own security".

Hammond added: "Ultimately the best opportunity for a stable and secure Afghanistan for the long term lies in a political settlement. 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."

Hammond acknowledged he could not protect all of the troops who would serve in Afghanistan over the next two years from forthcoming redundancy rounds – the army has to shrink to 82,000 because of defence cuts. He also said the military would not bring everything home from Afghanistan. Some equipment would be left to the Afghans, or neighbouring countries, and some equipment would be destroyed.