Dr Sellers, who is originally from Crowborough, in East Sussex, was due to undertake the journey on the Endeavour rocket in February 2004.

But following the loss of the orbiter Columbia on Saturday - which disintegrated over Texas on re-entry, killing the seven astronauts on board - that start date could now be put back.

His father-in-law, Philip Lomas, said on Monday: "He's 99% certain the mission will go ahead but that it will be delayed.

It goes to show it's a dangerous business Dr Piers Sellers, Sussex astronaut

"He was going to be the chief space walker."

He said his son-in-law, 47, had trained with some of the astronauts who died.

Dr Sellers, a father-of-two, spoke on Saturday from his home in Houston, Texas, of his shock at hearing of the Columbia disaster.

"Three or four of my friends are on board," he said. "I never thought I would see this on entry. The attention for this sort of thing is usually focused on take-off.

"But it goes to show it's a dangerous business."

Dr Sellers became only the third Briton to go into space when he completed an 11-day mission to the International Space Station in October 2002.