Troop numbers are already being reduced from 9,500 to 9,000 before Christmas.
And numbers would fall to about 5,200 by the end of 2013, Mr Cameron told MPs at Prime Minister’s Questions.
All Nato operations are due to finish by the end of 2014, with responsibility being transferred to Afghan forces.
But a small number of British troops would remain in the country beyond that, working at an officer training academy and “involved in returning equipment and dealing with logistics”, Mr Cameron said.
Read more at BBC News