A budget fight in the United States Congress, played out in closed-door meetings and cryptic exchanges on the Senate floor, has revealed the existence of a previously top secret "stealth satellite" programme, designed to create a new generation of American spy satellites that cannot be detected from the ground.

The programme's existence has been known only to officials with the highest security clearances and senior senators and congressmen who sit on the committees that oversee intelligence budgets.

It might have stayed secret a while longer, except for the growing vehemence of the policy debate among congressmen and senators over the wisdom of an unproven project that is reported to have doubled in cost, and now takes up a quarter of the £21 billion US intelligence budget.

Last week, senior Democrats on the Senate intelligence committee refused to sign off on the 2005 intelligence budget in protest at a programme that they believe will have little use against terrorists shopping for banned weapons or North Korean and Iranian nuclear facilities, which are designed to be invisible from space.

Attempting to explain why he was staging his protest, the senior Democrat on the committee, Senator John Rockefeller, could only tell colleagues that he had a "strenuous objection to a particular major funding acquisition programme that I believe is totally unjustified and very wasteful and dangerous to national security".

Conservatives denounced Mr Rockefeller for allegedly giving away a state secret, but he insisted that his remarks had been vetted by intelligence officials beforehand.

The satellite system is reportedly designed to replace an existing stealth satellite, codenamed Misty.

It would string satellites in the sky that would look like space debris to nations trying to spot US surveillance satellites.