Britain's first national security adviser is to leave his post, the government confirmed Monday -- less than three months after the official took up the newly created role.

Peter Ricketts, previously the country's top diplomat, was appointed after Prime Minister David Cameron's government took office in May to lead its security work and set up a new national security council.

A spokeswoman for Cameron said Ricketts would step down in 2011, and insisted that he had only ever intended to serve in an interim capacity.

"He is not leaving his post early," the spokeswoman said, on condition of anonymity in line with policy. "It was always the understanding that he would do it for a limited period."

She said it is not confirmed who will replace Ricketts as Cameron's chief security adviser.

Ricketts sits alongside Cameron, other ministers and the heads of the military and intelligence services on the security council, which is responsible for key decisions on the Afghanistan conflict and domestic security.

Cameron clashed last week with key allies in Pakistan over terrorism, after he questioned Islamabad's commitment to tackling terror groups inside its borders.

The spokeswoman insisted the departure of Ricketts was unconnected, and that Cameron had full confidence in the adviser's work.

Ricketts, who was previously head of Britain's joint intelligence committee and served as chief diplomat at the Foreign Office for four years, is expected to be offered a prestigious post overseas next year.

Cameron is likely to appoint new ambassadors to Paris, Washington and India over the next 12 months as incumbents reach the end of their postings.

Though the British leader has said he intends to break with tradition and appoint businessmen as ambassadors in some countries, Ricketts is expected to be rewarded with a post in the United States or France.

Britain's Foreign Office declined to comment. "Announcements about senior positions will be made at the appropriate time," the ministry said in a statement.