* Tough choices ahead for UK military, document says
* Call for greater European cooperation
(Adds new Ainsworth quote, Conservative comment)
By Adrian Croft
LONDON, Feb 3 (Reuters) - Britain said on Wednesday its armed forces faced tough choices to adapt to an age of austerity but said greater defence cooperation, including with other European nations, could help make stretched budgets go further.
A government discussion paper on the future of the military acknowledged Britain could not afford to pursue all of its current defence activities while supporting operations in Afghanistan and elsewhere and investing in new systems.
"We will need to do things differently in the future and prioritise some activities over others. Hard choices and important decisions lie ahead," Defence Secretary Bob Ainsworth wrote in the paper.
Britain, a staunch ally of the United States, has one of Europe's strongest armed forces but it has been stretched by the wars in Iraq and Afghanistan, where it still has 9,500 troops.
The government's need to rein in a deficit forecast to reach 178 billion pounds ($285 billion) this year has raised questions over whether it can afford all the multi-billion-dollar equipment programmes it has signed up to, ranging from a replacement for its nuclear-armed submarine fleet to two new aircraft carriers to the Joint Strike Fighter.
The Afghan and Iraq conflicts have highlighted failings in equipment such as a lack of helicopters and armoured vehicles.
The paper, intended to form the basis for a review of defence strategy, made no recommendations on equipment.
But Ainsworth said the Labour government stood by its decision to replace the nuclear submarines, expected to cost 20 billion pounds, and was committed to the new aircraft carriers.
CONSERVATIVES MAY TAKE DECISIONS
Critical decisions on the future of the British armed forces may fall to the opposition Conservatives, favourites to win an election expected in May. They too have promised to conduct a strategic defence review, if elected.
Ainsworth told parliament "not a penny will be cut" from next year's defence budget but that, in future, it faced "real financial pressure". One way to make best use of the available resources was greater international cooperation, he said.
NATO remained the cornerstone of Britain's security, the paper said, stressing the importance of its U.S. ties.
"However, as Europeans, we must take greater responsibility for our security together. Stronger European defence cooperation offers many opportunities, not least in the wider role defence should play in resolving conflict and building peace," it said.
France's return to NATO's integrated command structure last year offered an opportunity for greater cooperation, it said.
Conservative defence spokesman Liam Fox agreed France and the United States were likely to be Britain's main strategic partners, but he voiced doubts about other European allies.
"For us there are two tests: Do they invest in defence? And do they fight? Too few European allies pass both these tests," he said.
Bastian Giegerich, research fellow for European security at the International Institute for Strategic Studies thinktank, said defence cooperation had been forced on to the agenda by "the continuing high demand on forces, the consistent capability gap and the budget crunch."
Current EU defence cooperation includes a naval force set up to combat piracy off Somalia. Britain has in the past been suspicious about duplicating NATO functions within the EU for fear it would undermine the transatlantic alliance.