Insulating foam on the external fuel tank was damaged by a February hailstorm that left over 2600 divots. Some can be repaired simply by sanding the area and others can be filled by pouring liquid foam into the dents.

Around half the divots are in clusters that require the wholesale removal of the damaged insulation and then new foam sprayed onto the tank, a job normally done by robots in a New Orleans plant rather than by technicians at Kennedy Space Center.

"What we're doing is letting the work drive the schedule, not the other way around," said Wayne Hale, manager of NASA's shuttle program.

Current progress means the repairs could be completed and the shuttle rolled out to the launch pad by May 6, but the launch window runs from June 8 to July 18. NASA officials had previously hoped to launch Atlantis in May.

The delay means astronaut Sunita Williams' stay on the space station will extend to eight months, setting a new US record in the process. "It really doesn't matter, I have lots to do up here," Williams said. If the Atlantis launch is further delayed into July, NASA will consider bringing her back on that flight rather than have her wait for the following shuttle mission as currently planned.