Tom Hedley agreed to sign for the Rebels in January

Hedley has struggled to recover from a fall at Sheffield last year, which was compounded by another in his first meeting back in his native Australia.

"This comes as a complete shock," said Rebels promoter Mike Golding. "We expected big things from Tom."

The Rebels have less than three weeks to find a replacement in time for the start of the 2008 season.

Hedley had appeared to be on the way back, riding in all five rounds of the Australian Championships and their Under-21 championship, albeit with moderate success.

He admitted to being disappointed with those results, but had indicated he was looking forward to returning to England and the new challenge at Somerset.

But Hedley then telephoned Mike Golding, and said he had no confidence in his riding and would not be returning to the UK for the 2008 season.

This leaves Somerset with little time to find a rider whose average fits the 6.19 available, but the club say that every avenue is being explored.

"We sorted everything very early this winter and now we have lost two riders before the start of the British season," said Golding, referring to their Ritchie Hawkins who will miss the season after being injured at an indoor meeting in Germany at the end of January.