The British government's plans to expand London Heathrow Airport were grounded Friday as a judge ruled they must be reconsidered, to the delight of environmental campaigners.

Heathrow, west of London, is the world's busiest international passenger airport. Britain's main air hub is operating at capacity and the government says a third runway is crucial to boosting the British economy.

But judge Robert Carnwath at the High Court in London upheld campaigners' claims that the policy needed to be reviewed in respect of how it tallies with the government's environmental pledges.

The government's third runway plans are now "in tatters", the coalition of local councils, environmental groups and residents said in a joint statement.

"If the government wants to pursue its plans for Heathrow expansion, it must now go back to square one and reconsider the entire case for the runway," it said.

But the government said they would not be changing their plans.

Heathrow is "vital" to the national economy and expansion was needed "to secure jobs and underpin economic growth", Prime Minister Gordon Brown said at a European Union summit in Brussels.

"It is entirely compatible with our carbon reduction targets, as demonstrated by the recent report by the independent committee on climate change.

"We took a tough decision, the right decision necessary for the future of Britain and the economy, and a new runway will help secure Britain's economic future."

A Department for Transport spokesman said: "Today's judgment in fact upholds our policy and refused to quash our decisions -- it changes nothing."

In his ruling, ordering a reconsideration, Carnwath said: "Whether there should be a third runway at Heathrow Airport is a question of national importance and acute political controversy.

"It... may well become a significant debating point at the forthcoming general election."

He adjourned the hearing for more than a week to give both sides time to consider what orders the court should make on the basis of the judgment.

The main opposition Conservatives, who are leading in the polls ahead of an expected May 6 general election, are opposed to a third runway at Heathrow.

"There is no case for it on environmental grounds, there is no strong business case for it," party leader David Cameron said.