British Prime Minister David Cameron on Wednesday said that he will withdraw almost half of the country's 9,000 troops from Afghanistan next year as NATO hands over to Afghan forces.

The announcement comes as NATO prepares for a full security handover at the end of 2014, despite fears that a civil war could follow, and amid a spike in "insider attacks" on foreign troops by Afghans in uniform.

Karzai said the decision was "well-timed" and insisted his war-scarred nation was ready to take charge of its own security.

"The president... welcoming the announcement said: 'the Afghan national forces are ready to provide the security and defend their country," a statement from Karzai's office said.

"The decision by Britain is a well-timed decision."

Cameron said the withdrawal of around 3,800 British troops by the end of 2013 was possible "because of the success of our forces and the Afghan National Security Forces".

There are currently more than 9,000 British troops serving in Afghanistan with the NATO force -- the second largest force in the country after the United States.