Tony Blair has been accused of weak leadership after American plans for a national missile defence system caused an apparent difference in emphasis between the prime minister and his official spokesman, Alastair Campbell.

Questioned in the Commons on Wednesday, Blair repeated the established government line that the US government had not yet put forward a specific proposal and when it did so, the government would consider it carefully.

However twenty minutes later Campbell was asked by journalists if he thought missile defence was "a good idea" and responded: "Broadly, yes, in that the Americans are the last remaining superpower. They are the only ones who can develop this new technology and they are going ahead with it."

The Conservatives kept up the pressure in the Commons on Thursday, attacking the government for being divided and giving more importance to journalists than MPs.

Francis Maude, shadow foreign secretary, said the House was "entitled to feel extremely angry that it is treated in this way."

It was important "for the British government to speak with a single voice on the issue" to maintain influence with both America and Europe, said Maude, who also called for the government to support missile defence.

Foreign secretary Robin Cook emphasised that the government's position had not changed, saying he was "entirely happy" to endorse the prime minister's statement that America needs to put forward detailed plans before the government can take any decision.

Cook also accused the Conservatives of a "betrayal of the national interest" in agreeing to American plans before they knew what they were.

Responding to a claim from Tony Benn that the government was being cautious ahead of the general election because of widespread opposition, Cook said the government would be "cautious, responsible and realistic" after the election.

The British government's position on national missile defence, dubbed the "Son of Star Wars", remains important as it would have to consent to an upgrading of radar stations in Yorkshire for American plans to proceed. The government could yet be forced to chose between offending either the American administration or European allies.

American president George W Bush said on Tuesday that "We are not presenting our friends and allies with unilateral decisions already made" and added that high-level representatives would tour the capitals of key allies next week.

However Bush also said: "When ready, and working with Congress, we will deploy missile defences to strengthen global security and stability."

As America prepares to push ahead with plans, other European government's remain sceptical while Russia has moved to a more conciliatory approach. The Chinese government has warned of a news arms race if the US violates the 1972 Anti-Ballistic Missile treaty.