David Cameron is expected to signal a stepping up of Britain's withdrawal from Afghanistan amid speculation that the force size could be nearly halved next year.

The Prime Minister is set to outline the Government's latest thinking to MPs after agreeing with US president Barack Obama that there are "further opportunities" for personnel to be brought home over the next 12 months.

The two leaders covered the situation in an hour-long video call last night and believe that the plan for all combat troops to leave by the end of 2014 is "on track".

"On Afghanistan, they discussed progress on the plan to hand security responsibility from the International Security Assistance Force (ISAF) to the Afghan National Security Forces, and agreed that the Nato strategy to withdraw combat troops from Afghanistan by the end of 2014 was on track," Downing Street said.

"This would present further opportunities for ISAF countries to bring troops home next year and they agreed to stay in close touch as detailed plans develop. 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."

UK troop numbers are already being reduced from 9,500 to 9,000 before Christmas. And that could fall to around 5,000 after next summer's fighting season.

ISAF commanders have been delivering optimistic reports about progress by Afghan security forces. But there have also been claims that investors are pulling out of the country over fears that Taliban insurgents will resurface when Western troops leave.

Mr Cameron is expected to take the opportunity to refer to operations in Afghanistan during Prime Minister's Questions. It is understood he will give a clear indication of force strength for next year after the National Security Council reached consensus.

Defence Secretary Philip Hammond is then slated to deliver a quarterly update on Afghanistan to the Commons.

The US currently has around 60,000 troops in Afghanistan, after withdrawing some 23,000 this year.