Labour chairman of the Commons Foreign Affairs Committee, Donald Anderson, insisted that what Mr Campbell had said was "still a long way from William Hague's blank cheque" of endorsing the US position.
The spokesman, Alastair Campbell, told reporters that the planned "Son of Star Wars" missile shield was "broadly a good idea".
His comment came only minutes after Tony Blair had refused to be drawn on the issue in the Commons.
This prompted shadow defence secretary Iain Duncan Smith to ask whether it was the prime minister's official spokeman and not Mr Blair who was running the country.
And Liberal Democrat defence spokesman Menzies Campbell asked why, if the government believed the National Missile Defence system (NMD) was a good idea, Mr Blair had not told the Commons?
Joint statement
But Downing Street dismissed the attacks as a "fuss about nothing".
Another spokesman for the prime minister said a joint statement agreed by President Bush and Tony Blair at Camp David in February was evidence that the governmen'ts position had always been clear.
"Nothing was said this afternoon which takes us into a different position," he said.
But Mr Duncan Smith told the BBC: "It is breathtaking.
"It must be about the most serious decision that any government can make."
He said the decision was one that should rest with the prime minister - but it appeared to have been Alastair Campbell who had decided to change the government's position.
He went on: "Who's governing the country for goodness sake?"
"So close to an election, it is obvious that Tony Blair doesn't dare annoy all the card-carrying CND members on the Labour front and back benches - so he's leaving his dirty work to the spin doctors as usual."
'Anxieties'
For the Lib Dems, Menzies Campbell said there were "very considerable anxieties about NMD".
"Only the British government now, of all the European allies in Nato, thinks NMD is a 'good idea'.
"What price cooperation with our European allies now?"
Labour chairman of the Commons Foreign Affairs Committee, Donald Anderson, insisted that what Mr Campbell had said was "still a long way from William Hague's blank cheque" of endorsing the US position.
But he said he thought the more cautious line taken by Mr Blair during prime minister's questions was preferable.
'Crazy Star Wars'
A spokeswoman for Greenpeace said: "Tony Blair is looking increasingly like a puppet whose strings are being pulled first by Bush and then by his press spokesman.
"If he was a strong leader he would say no to Bush's crazy star wars plan."
Earlier in the Commons, Mr Blair had insisted that he would not be drawn into a decision until a "firm proposal" was on the table.
He said: "It is important and right that we wait for a firm proposal before giving a firm decision."
Mr Blair was speaking after the US president's announcement on Tuesday that he was committed to building a global missile defence shield, in breach of the 1972 Anti-Ballistic Missile treaty.
That prompted former Tory prime minister Lady Thatcher to come out strongly in favour of the proposal - and to challenge Mr Blair to do the same.
She told the prime minister to stop "shilly-shallying" and to champion the president's "bold vision" in every international forum.
But former Labour defence secretary and chancellor Lord Healey told BBC Radio 4's World Tonight programme that no one with "common sense" would back the US plans.
"The real reason the government is supporting the scheme is that it wants to maintain good relations with the United States."
He added: "The plain fact is it is a total waste of American money and I can't think why anybody with common sense would support it."
Defence Secretary Geoff Hoon told the same programme that NMD was a "good idea" for America and ministers understood why President Bush was pushing ahead with it.