By Dr David Whitehouse BBC News Online science editor

The US space agency (Nasa) has grounded its space shuttle fleet indefinitely after finding small cracks in fuel pipes in the main engines of two orbiters.

The cracks, discovered on Atlantis and Discovery, will delay the scheduled 19 July launch of Columbia, due to carry the first Israeli astronaut, Ilan Ramon, into space.

The cracks were found in metal liners used to direct fuel flow inside the main rocket motor's fuel pipes.

"The concern is that... if a piece were to crack off and go down into the engine, would that damage the engine and cause it to shut down?" said Nasa spokeman James Hartsfield. "Whether that is something that could actually happen, we don't know yet."

Safety concern

"These cracks may pose a safety concern and we have teams at work investigating all aspects of the situation," said Ron Dittemore, Nasa's shuttle programme manager.

"This is a very complex issue and it is early in the analysis. Right now there are more questions than answers."

Engineers found the first crack during an inspection of "flow liners" - thin pieces of metal that aid the flow in fuel pipes - as they were installing engines in Atlantis more than a week ago.

Subsequent testing found other cracks in both Atlantis, which is 17 years old, and Discovery, which is 19. Columbia, the oldest of the four shuttles in the fleet at 21 years old, is to be inspected.

Some of the cracks are as small as 0.25cm (0.1 inches)

Because it takes a week or so to remove a shuttle's engines and the same amount of time to reinstall them, Columbia's forthcoming mission on 19 July has been delayed indefinitely.

Nasa engineers will try to determine if the problem is a flaw introduced at the time the flow liners were installed in the shuttles or whether the cracks are a problem of age.

"We've never seen these [cracks] before," said James Hartsfield. "The flow liners have been in the shuttles since day one. We have begun an analysis to try to understand it."