The news was confirmed to the Money Centre Stars supporters on Wednesday, and club officials were quick to praise Harrison's contribution to the team in this season's Premier League challenge.

Harrison has been diagnosed with deep vein thrombosis in his stomach, following tests at a hospital near his Workington home on Monday.

Team manager Rob Lyon said: "As it stands that means the end of Rusty's career. He's been told he must not take part in any impact sports and obviously speedway falls into that category.

"Rusty is going back for more tests, but he's already been told he must have Warfarin (an anti-coagulant medicine) for the rest of his life."

At the end of last season Harrison put his career on hold, but he was tempted back by Lynn during the winter when they needed a replacement for Trevor Harding, who had been badly injured in a crash in Australia.

In 29 league matches for the Money Centre Stars this season, Harrison scored 187 points for an average of 6.45.

He was taken to Lynn's Queen Elizabeth Hospital during last week's meeting with Isle of Wight for tests to a suspected blood clot to his leg.

At the time they were not conclusive and he was allowed to go back home for further tests to be carried out this week.

Lyon said: "Rusty came in at short notice and has been a great team man. He has done a solid job for us and I've enjoyed working with him.

"This is such a shame for Rusty and his family – and for anyone in that situation. I'm sure we will try and keep him involved in some way as he is still a part of our team.

"It's cruel, but as a club we have got to get on with it and start looking ahead to the play-offs."

Club promoter Jonathan Chapman added: "This has been sad news for Rusty, for the club and for speedway.

"He had done a brilliant job for us and we were looking to bringing him back next season."

A joint collection for Harrison and Shaun Tacey – who is out with a broken collarbone – was held at the Norfolk Arena on Wednesday.

Tacey told the fans: "You've been very generous and on behalf of Rusty it's a big thank-you to you all. This will help put a smile on his face."

The Stars wind up their regular season on Sunday with a visit to Mildenhall and have called up ex-club favourite Chris Mills as guest replacement for Tacey, while continuing to use rider-replacement as cover for Harrison.

Reading report: Page 126 of today's paper.