The twice-delayed launch of a Nasa space shuttle Discovery may now be called off altogether after engineers found a 5-inch crack in the insulation foam on the external fuel tank.

The crack was spotted during an overnight inspection. NASA had been hoping to launch Discovery tomorrow, the Fourth of July holiday in the United States, having already postponed launch plans on Saturday and Sunday because of bad weather.

NASA found the crack, about an eighth of an inch wide (3mm), in the foam on a bracket about two-thirds of the way up the external fuel tank, on the side facing the shuttle. That location would make it easy to hit the shuttle if a piece of foam came off.

"We don’t know if it’s a problem or not," said George Diller, the NASA spokesman.

Officials told a press conference tonight that the decision on whether or not to go ahead with tomorrow's launch would be delayed for five hours, while engineers examined the problem and tried to work out how serious it was.

Before the latest safety alert Michael Griffin, the NASA Administrator, had decided the shuttle should go into orbit despite the concerns of two top agency managers who wanted additional repairs to the foam insulation.

The space shuttle Columbia was brought down in 2003 by a chunk of flyaway foam that damaged its wing, and another piece of foam broke off Discovery’s redesigned tank last July, barely missing the shuttle.

One option being considered by NASA today was to sand down the area of foam, which would take a couple of hours and could be done on the launch pad. Another would be to cut out the area of foam and replace it, which could take several days, said Marion LaNasa Jr., a spokesman for Lockheed Martin Space Systems, which makes the tank. Mr LaNasa added that he did not know if foam had ever fallen from that area during previous launches.

If NASA decides to go ahead, it would be the first manned launch by the United States on the nation’s birthday, and only the second lift-off of a space shuttle since the 2003 Columbia disaster.

The mission for Discovery’s crew this time is to test shuttle-inspection techniques, deliver supplies to the international space station, and to drop off German astronaut Thomas Reiter for a six-month stay.

The weather forecast for liftoff tomorrow was better than it was for either Sunday or today, with a 40 perc ent chance that storms at launch time would prevent liftoff, said US Air Force 1st Lt. Kaleb Nordgren, a shuttle weather forecaster.

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