* UK Afghan force to fall to 5,200 from 9,000 next year
* PM says most soldiers home by end of 2014
* NATO mission ends 2014; Afghans to take security role
By Peter Griffiths and Matt Falloon
LONDON, Dec 19 (Reuters) - Britain will withdraw nearly half its troops from Afghanistan by the end of 2013, the government said on Wednesday, as part of a security handover to Afghan forces more than a decade after the U.S.-led invasion.
Nearly all of Britain's 9,000 soldiers are due to pull out when the NATO mission finishes in late 2014, ending a long, costly and unpopular war that has cost the lives of 438 UK troops.
Like the United States, Britain will leave behind an undisclosed number of soldiers after 2014 to help local forces face threats from the Taliban and its al Qaeda allies.
"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," Prime Minister David Cameron told parliament. He had discussed the plan with U.S. President Barack Obama by phone on Tuesday.
Britain, which has the second biggest foreign force in Afghanistan after the United States, says it has helped to stabilise the country and prevent militants from finding a safe haven.
But the war's critics say Afghanistan is far from stable after years of violence and they question why Britain has spent so much money on the war at a time of tight public finances.
Britain's defence budget, like that of other NATO members, is under pressure, forcing the Ministry of Defence to spend less on troop numbers and equipment.
Polls for the BBC and several UK newspapers over the last two years suggest a majority of British voters think the conflict is "unwinnable" and that its soldiers should leave Afghanistan ahead of schedule.
Some Afghans fear the withdrawal could lead to an escalation in violence or even civil war. However, Afghan President Hamid Karzai has dismissed those concerns.
A resilient Taliban, ethnic tensions and competing warlords all pose a threat to Afghanistan's security before presidential elections due in 2014. Attacks on NATO forces by rogue Afghan soldiers and police have raised tensions.
An Afghan defence ministry spokesman said its forces could fill the gap left by the withdrawal of foreign troops.
"There will not be any security gap and we are fully prepared to provide security," General Zahir Azimy told Reuters.
British Defence Secretary Philip Hammond sought to allay fears there would be a fast pullout that could threaten Afghanistan's security.
"There will not be a cliff-edge reduction in troop numbers at the end of 2014," he said. "There remain huge challenges ahead for the Afghan people. Our combat mission is drawing to a close, but our commitment to the Afghan people is long term."