British forces in Afghanistan will be reduced to just over half their current size by the end of next year as the exit strategy from the long and costly war gets under way.
The total UK contingent will be reduced from its current strength of 9,500 to around 5,000 after the end of the next summer's fighting season, it is expected, with further and escalating decreases taking place with each brigade sent to Helmand.
David Cameron will make the announcement today. Final details, however, are still being worked out with partners in Nato, especially the Americans.
The scale of British disengagement will depend to a large extent on what the US administration decides: the Americans, in turn, are said to be accelerating their pace of withdrawal after commanders on the ground claimed that the Afghan army and police are making faster progress than expected.
Barack Obama has indicated that he will make up his mind about American troop numbers by the end of the year. Diplomatic sources said that although the President is currently engaged in dealing with the Connecticut school massacre, he will have decided by the time he meets the Afghan President, Hamid Karzai, in Washington in early January.
The Americans currently have 60,000 troops in the country; 23,000 were withdrawn this year, following 11,000 in the previous 12 months. The US Defence Secretary, Leon Panetta, is said to have been persuaded towards a sizeable pull-out after consulting senior officers.
One commander, Major-General Robert Abrams, insisted that Afghan forces had made "astounding" gains in districts around Kandahar while another, Maj-Gen Larry Nicholson, maintained that the Taliban had failed to regain lost ground.
The Afghan view is different, with apprehension about the insurgents taking over large areas of the country when Western forces end combat operations in 2014, which Afghan commanders say may be too soon.
Nevertheless the British Prime Minister, who inherited a war he does not like, will seize on the American lead to reduce the UK presence in the conflict at a faster rate, Whitehall sources say.
Some 438 members of British forces have been killed in Afghanistan. The Chancellor and other ministers opposed to the war point out that it is costing £2.5bn a year.