Relations between NATO and the European Union are "plagued by mistrust and unhealthy competition", according to a committee of MPs.

The Commons defence committee warned that European defence policy risked undermining the US's commitment to the alliance.

In a report published on Thursday ahead of next month's NATO summit in Bucharest, the MPs said European governments did not have enough "political will".

Describing NATO as "indispensable", the committee said it was the "ultimate guarantor of our collective security" in Europe.

"Without US support, NATO has no future," said the report. "But US support depends on NATO becoming more capable, deployable and flexible, and on the European allies contributing more."

It stressed the importance of operations in Afghanistan, claiming that failure in the country would "deal a severe blow to allied unity".

The report claimed that the Europeans displayed a clear lack of political will in failing to spend more on defence, with only six of the 24 member states meeting pledges to spend at least two per cent on defence.

"If the European members of the alliance want to be taken seriously, if they want the United States to remain engaged in, and committed to, NATO, and if they want greater influence in the overall direction of alliance policy, they must commit the necessary resources and improve their capabilities," it added.

"We are concerned that an alliance with such large and growing discrepancies in defence spending will not be sustainable in the long term."