Britain will announce a new withdrawal of troops from Afghanistan on Wednesday, officials said, with media reports saying that around 4,000 soldiers are to be brought home next year.
Prime Minister David Cameron is expected to outline plans to lawmakers in parliament that would nearly halve Britain's force in Afghanistan, which is currently around 9,000-strong.
A Downing Street source confirmed to AFP that an announcement would be made on a troop withdrawal on Wednesday but would not give details of numbers.
Britain has already said it intends to pull out all its combat troops by the end of 2014 and hand over security responsibility from the NATO-led International Security Assistance Force (ISAF) to Afghan forces.
But the government has faced growing pressure at home to speed up the withdrawal from Afghanistan, where Britain has been fighting since 2001, following a series of "insider attacks".
Cameron discussed the plans with US President Barack Obama in an hour-long video call on Tuesday night during which they agreed the handover plan was "on track", Downing Street said in a statement.
"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," the statement said.
"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 United Kingdom."
The final decision on the latest withdrawal was taken at a meeting of Britain's National Security Council on Tuesday morning, reports said.
Cameron is expected to speak on the issue during prime minister's questions on Wednesday and Defence Secretary Philip Hammond is due to give a statement on Afghanistan afterwards.
British newspapers and broadcasters said around 4,000 troops would be pulled out over the course of 2013, but that levels were likely to be kept at around 5,000 until the handover at the end of the following year.
The government is considering leaving several hundred British troops in Kabul for training and support roles when NATO ends its combat role in 2014.
Britain has already withdrawn 500 troops from Afghanistan over the course of 2012.
Britain has lost 438 troops in Afghanistan since operations began in October 2001. Of these, at least 395 were killed as a result of hostile action.
The US military currently has about 66,000 troops on the ground, as part of a NATO-led force of roughly 100,000.
The British announcement comes as Kabul laid out a five-step plan that could bring hardline Taliban Islamists into government as efforts to broker peace accelerate ahead of the withdrawal of Western troops.
The Taliban regime was ousted by a US-led invasion in 2001 and there are concerns that their return to any sort of power could see an erosion of gains in democracy and human rights, particularly the rights of women.