LONDON, June 30 (Xinhua) -- British Defense Secretary Liam Fox warned on Wednesday that British troops may be the last to leave Afghanistan as their mission in the country is one of the most difficult ones.

Fox told BBC in Washington that Helmand, where British troops are based, was likely to be one of the last places to be transferred to Afghan charge.

The strategy should be given time to work "without constantly trying to change and question it," he added.

In a speech in Washington, Fox emphasized that it is important for people to keep their nerve about the mission and to leave before the job was done would send a message to the enemy.

"It would send the signal that we did not have the moral resolve and political fortitude to see through what we ourselves have described as a national security imperative," he said.

His comments come days after British Prime Minister David Cameron said he wanted to see British troops withdraw by the next general election, due in 2015.