British troops must never come under the control of the European Union, says Liam Fox.

In a keynote speech on Conservative policy, the shadow defence secretary also said that NATO must retain a key role in British security policy.

He was set to tell the Centre for Policy Studies on Monday evening that events in the Balkans had exposed a gap between "EU rhetoric and the ability to act effectively".

"Indeed, the Yugoslav crisis was a salutary lesson in the limits of European integration, and specifically in the difficulty of sharing sovereignty in the sensitive areas of security and defence," warned Fox.

And he said the issue of democratic accountability was "truly insurmountable".

"The decision of any government to commit its troops to combat is perhaps the most difficult, important and serious decision that can be taken," said the Conservative spokesman.

"For these reasons that decision must always be made in the full knowledge that the government will, ultimately, be held accountable to the electorate.

"So Britain can never allow its troops to be sent into action by any supra-national body, still less one with no democratic accountability.

"There is not, and cannot be, a role for the European Union in this sphere."

Fox added that "the only increase in overall EU capability has been the establishment of institutions".

"Common defence policy is the only relic of the rejected constitution which is still being progressed determinedly by Brussels," he said.

"Some in Europe envisage the creation of a multi-polar world in which the EU acts as some sort of counterbalance to the military might of the US.

"This overlooks the fact that the US has been the main and successful guarantor of our security in the post-war world.

"In Britain, we also benefit not only from co-operation over the nuclear deterrent but also from the sharing of intelligence.

"Bilateral Anglo-French procurement projects for aircraft carriers, or Anglo-German-Italian-Spanish aviation projects such as Eurofighter, or Anglo-Dutch amphibious troops, are in our national interest.

"But I see no need to involve the EU given such bilateral projects work well – and better than many EU projects and missions."