By Phil Stewart and Andrea Shalal-Esa

WASHINGTON (Reuters) - Deputy Defence Secretary Ashton Carter, known for his deep knowledge of U.S. defence spending and the defence industry, said on Thursday he was stepping down in December after four years in top Pentagon jobs.

Defence Secretary Chuck Hagel said he "reluctantly accepted" Carter's decision to leave the post.

It was unclear who might replace Carter, although several names surfaced late Thursday as possible successors: Navy Secretary Ray Mabus, former Air Force Secretary Michael Donley and the Pentagon's former policy chief, Michele Flournoy.

Another possible contender might be Linda Hudson, a long-time defence industry executive who has announced plans to retire early next year as chief executive of BAE Systems Inc, the U.S. unit of Britain's BAE Plc.

As deputy defence secretary over the past two years, Carter, who has a doctorate in theoretical physics from Oxford University, helped ensure a smooth hand-off from then Defence Secretary Leon Panetta to Hagel.

"He possesses an unparalleled knowledge of every facet of America's defence enterprise, having worked directly and indirectly for eleven secretaries of defence over the course of his storied career," Hagel said in a statement.

Carter, a long-time Harvard University professor, himself had been rumoured to be a contender for the defence secretary job before Hagel's name emerged as the top candidate. Some speculated he could be a possible head of the Department of Energy.

Loren Thompson, a Virginia-based defence consultant, said Carter's departure robbed the Pentagon of an experienced manager and could also open the door for mergers among bigger defence companies.

"Carter was the policymaker who said no mergers between top-tier defence contractors. That prohibition is likely to leave with him," he said.

"At the rate demand is declining, there is not going to be enough business for some of the bigger players in information technology, electronics and other defence sectors."

Before becoming deputy defence secretary, Carter was the defence undersecretary for acquisition, technology and logistics, wrangling with complicated weapons programs such as Lockheed Martin Corp's F-35 fighter program.

His deputy and successor in that job, Frank Kendall, was also mentioned as a possible contender for the No. 2 position. Kendall is seen as a strong, no-nonsense inside manager, and someone whom Hagel likes and respects.

Carter said he had long planned to step down on December 4 but delayed his announcement because of financial uncertainty facing the Defence Department, which has been affected by the partial government shutdown that started on October 1 and across-the-board budget cuts that forced it to put civilian employees on unpaid leave this summer.

"But I have decided that this situation might well continue and I don't want any more time to pass before giving you the opportunity to begin a smooth transition," Carter said in his resignation letter to Hagel.

Hagel and other senior officials gave Carter a standing ovation at the meeting where his departure was announced.

"He'll be missed by the secretary. They've had a strong and effective working relationship and friendship that will continue," said Pentagon spokesman George Little. "There's a lot of work to do between now and December 4."

(Additional reporting by David Alexander; Editing by James Dalgleish and Mohammad Zargham)