A violent hailstorm caused over 150 dents in the external fuel tank's insulation. Nasa engineers have decided to take the shuttle back from the launch pad to a repair area.

"The earliest we could launch now would be 27 May," said Nasa spokesman Bruce Buckingham. The planned date had been 20 May.

Space shuttle Discovery's planned mission is to the International Space Station (ISS), to deliver tonnes of supplies and erect a crane for future construction.

Deep dents

The dents were found in the foam insulation covering the shuttle's external fuel tank. The worst were five centimetres (two inches) deep.

Nasa is worried that the super-cold fuel in the tank could make ice form in the craters in the tank's insulation. Then, after launch, the ice could break loose and hit the crew compartment with enough force to shatter a window.

Nasa said on Tuesday that the repairs could be done on the launch pad, without delaying the liftoff. But the ground team decided on Wednesday that would not be possible.

"We don't have access to much of the tank when the shuttle is on the launch pad," said Nasa's Lisa Malone at the Kennedy Space Centre. "When we get it back to the shuttle processing area, we can reach everything."

About 35 of the dents are believed to have been out of reach to the team on the launch pad.

Discovery's 10-day mission is the first since Endeavour and its crew linked the first two elements of the ISS last December. The five month hiatus is the longest in more than a decade.