* Britain holds defence review in 2015, election year
* Parliament's defence committee backs military spending
* Questions remain over future of nuclear weapons system
By Kylie MacLellan
LONDON, March 27 (Reuters) - Events in Ukraine have shown Britain must not cut its armed forces too deeply, a committee of lawmakers said on Thursday, saying it was concerned that allies had begun to question the country's military capabilities.
Britain has, over the last few years, been shrinking its armed forces by around a sixth as part of a plan to reduce the public debt, but the scale of the cuts has fuelled a debate about its ability to project force globally.
"Recent events in Ukraine illustrate the speed with which new threats, and indeed the reappearance of old threats, can manifest themselves," said James Arbuthnot, chairman of the Defence Committee, which scrutinises government defence policy and expenditure.
"Strong conventional forces provide the UK with a contingency against the unexpected threats that may emerge ... Events might require the reconstitution of conventional forces, but once cut back they will be very difficult to rebuild."
Britain is due to hold its next strategic defence and security review (SDSR) in 2015, the year of a national election.
The committee said operating a nuclear deterrent did not remove the need for substantial investment in other security measures, including diplomatic and intelligence assets as well as retaining adequate conventional forces.
"Deterrence must be credible to be effective: Britain has to show the capacity and the will to respond proportionately and effectively to threats at every level," said Arbuthnot.
Security strategy has been made more complex by threats such as cyber attacks, where identifying the enemy is harder, the committee said, increasing the need for focus on resilience to and recovery from potential attacks.
British generals have warned the military cuts, which come as the country prepares to withdraw the last of its troops from Afghanistan at the end of 2014, are hollowing out the armed forces and could limit Britain's ability to fight wars.
In January, former U.S. Defence Secretary Robert Gates warned British defence cuts meant the country could no longer be a full military partner of the United States.
The committee said it was concerned about these comments and the deterioration in perceptions abroad of the UK's military capabilities. It also said NATO remained a cornerstone of UK deterrence and said next year's defence review should focus on how best the UK can contribute to the alliance.
Britain is due to make a final decision in 2016 on whether to renew its Trident nuclear deterrent, an issue which has divided the coalition government.
The committee said the decision about retaining a nuclear deterrent should be made on its own merits, rather than on the basis of what Britain could do with the money saved.
"It would be naive to assume that a decision not to invest in the nuclear deterrent would release substantial funds for investment in other forms of security," it said. (Editing by Robin Pomeroy)