The premier, who was touring Camp Bastion in Helmand Province, said he was confident that the draw-down of nearly half of British troops, announced in parliament on Wednesday, would not pave the way for an increase in terrorist activity.
"The fact is they (Afghan security forces) are doing better than expected," he said.
"This is withdrawal. This is draw-down based on success, not on failure."
He added that the move was "being done for good military reasons and it has been done in a proper way.
"We're confident it can be done while making sure Afghanistan does not return to become a haven of terrorism which is of course why we came here in the first place," stressed the prime minister.
Britain has the second largest force in Afghanistan after the United States and has lost 438 troops in Afghanistan since the operation began to topple the Taliban in October 2001 following the 9/11 attacks.