Inspectors have found a fourth crack in support beams on the external fuel tanks of the space shuttle Discovery, whose final flight was delayed because of a hydrogen leak, Nasa said late on Monday.

The latest crack was found while technicians were removing foam while inspecting the support beams, which are called stringers, in the aftermath of the hydrogen leak, the space agency said.

Two 21-centimetre cracks were found in an adjacent support beam, and a 7.5-centimetre fissure was uncovered over the weekend.

“Further foam removal revealed one additional corresponding crack on the same left-hand adjacent stringer,” Nasa said.

“Technicians plan to remove that section of the stringer Monday night. They’ll also install a new section of metal, called a doubler because it’s twice as thick as the original stringer metal, on the stringer that had the nine-inch cracks,” it said.

Stringers are 6.4-metres long support beams in an area between the lower part of the external fuel tanks, which holds liquid hydrogen, and the upper part containing liquid oxygen.

The space agency is still aiming to launch Discovery at the next opportunity November 30, but senior managers will meet at the Johnson Space Centre on Monday to review the repair work and launch preparations.

To get in a flight to the International Space Station this year, Discovery must blast off before December 6. Otherwise it will have to wait until February, the same month that the last-ever shuttle launch is scheduled before the fleet is mothballed for good.

Discovery’s 11-day mission with its all-American crew of six is to deliver a pressurised logistics module called Leonardo to the ISS, which will be permanently attached to the space station to provide more storage space.

The shuttle will also bring Robonaut 2, the first human-like robot in space and a permanent addition to the orbiting space station, as well as spare parts.