Britain has an "open mind" on America's controversial national missile defence plan, Tony Blair has said.

In an unstable world threatened by rogue states and nuclear proliferation the prime minister said the scheme, which is strongly backed by George W Bush, has "merit".

"I've an open mind as to what missile defence can deliver us," he said.

"I think it's important that if we do play our part in a missile defence system that we make sure this country gets some benefit out of it."

But there is deep unease among Labour MPs about the "son of star wars" plan.

Many regard the move as an indicator of the US's increasing dominance on the world stage.

Peter Kilfoyle, the former defence minister, last week claimed the government was listening to "crackpot" ideologues in Washington.

The prime minister stressed that the proposals were defensive not offensive.

"The threat to US or indeed European security is unlikely to come from Russia or China but there is a threat to our security from unstable states acquiring nuclear weapons," he said.

"And if you can develop a defensive system, and this is a defensive system, that can give us some protection against that I don't think that is the wrong to do. On the contrary I think there is merit in it."

The real issues were technological not political, Blair told MPs.

"The questions I would have would be to do with the technology, whether it can really be developed in that way, and so forth," he said.