By Helen Briggs BBC News Online science reporter

The take-off has been put back by a week or so to allow electrical repairs to be made to the identical rovers.

The rover will act as a 'robot geologist'

Pre-launch tests at the weekend raised concerns that they might be vulnerable to a computer glitch.

The spacecraft will now have to be taken apart at Nasa's Kennedy Space Center in Florida.

George Diller of the Kennedy Space Center said an electrical circuit board needed to be modified.

"It'll be a fairly easy thing to fix but it does mean going into both of the rovers, removing the circuit card and making some modifications to it," he told BBC News Online.

The first Mars Exploration Rover was set to be launched by a Delta II rocket on 30 May.

It will now take off between 6 June and 19 June. The second spacecraft will be launched 10 days after the first.

Early June now looks like a busy time for leaving the Earth for Mars. The European Space Agency's (Esa) Mars Express spacecraft is also scheduled to lift off around 6 June. All the spacecraft should arrive at the Red Planet in early January.

Mars Express will orbit Mars and drop a lander, Beagle 2, on to the surface.

Bouncing ball

The Nasa spacecraft will touchdown on the Martian soil, releasing rovers capable of exploring the area.

"It will land like a big bouncing beach ball," explained George Diller.

"After it rolls to a stop the airbags will fall away from the spacecraft and then the petals of the lander will deploy like the opening of a rose.

"The rover will drive off one of the petals on to the Martian surface."

The two space agencies are both taking advantage of a line-up in the heavens that makes travel from the Earth to Mars relatively easy.

Nasa does not see it as a race. "What Beagle is doing is very complementary to what the Mars rovers are going to do and that's a good thing," he said.