LONDON — Britain will withdraw nearly half of its 9,000 soldiers from Afghanistan by the end of next year as Afghan national forces take on a bigger role, Prime Minister David Cameron said on Wednesday.

Nearly all of Britain's soldiers are due to pull out by the end of 2014, drawing to an end a long, costly and unpopular war that has cost the lives of more than 430 British troops.

"Because of the success of our forces and the Afghan national security forces . . . we'll 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 to the 9,000 we have now," Cameron told parliament.

Britain, which has the second biggest foreign contingent in Afghanistan after the United States, says its involvement has helped to stabilize the country and has prevented terrorists from finding a safe haven.

But the war's critics say the country is less stable after more than a decade of violence and they question why Britain has spent so much money on the campaign at a time of strained public finances at home.

Britain's defense budget, like that of other NATO members, is under pressure, forcing the Defense Ministry to spend less on troop numbers and equipment.

Ties between Western troops and Afghan forces have been tested by a series of "insider" attacks against NATO coalition troops by Afghan soldiers or by militants wearing Afghan military uniform.

After 2014, Britain will keep a small amount of soldiers in Afghanistan to help train Afghan forces, Cameron added.