Joonas Kylmakorpi is out of the rest of the speedway season with a back injury.
The Coventry Bees rider posted on Instagram: "The injury isn't bad, I have sustained disc compression but the recovery will take four to six weeks.
"It's very disappointing to resign while all my teams have great chances for league titles and I'm currently leading the Long Track World Championship.
"Getting back on the bike would bring greater risk of bigger injury and I'm not willing to take it.
"So far my season has been the best in my career, I have been one of the top scorers in each of my league teams, and won all Long Track Grand Prix rounds so far.
"Thanks a lot for your support, my winter training programme starts in October as planned."