Racing for an early exit from Afghanistan would fuel the risk of civil war in the nation, a senior Foreign Office official warned last night.

Mark Sedwill said "loose talk" about withdrawal before 2014 would make the possibility of the worst case scenarios becoming a reality more likely.

The Prime Minister's special representative for Afghanistan dismissed suggestions made earlier this month by former Liberal Democrat leader Lord Ashdown that British forces must withdraw as quickly as possible before any more troops are killed.

Mr Sedwill told the Commons Defence Committee: "If we raced for the exit then all of the fears that we have about Afghanistan's future would become that much likelier."

Asked if there was a risk of civil war he replied: "Yes, of course there is a risk.

"Loose talk about rushing for the exit in newspaper articles and books actually has a political affect because it undermines Afghan and Pakistani and regional confidence in our determination to see it through.

"If they act on their fears, then the worst scenarios become much likelier."

Most "green on blue" killings in Afghanistan are spur of the moment attacks rather than planned insurgency, MPs were also told.

Military chiefs told the committee only a "tiny number" involved planning, when quizzed about the surge in numbers of British deaths at the hands of Afghan soldiers or police.