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Watch Chambers' rugby league debut
Castleford have decided against offering sprinter Dwain Chambers a contract after the end of his trial.
Chambers tried his hand at rugby league after being frozen out of athletics following a two-year drugs ban.
But the 30-year-old, who had never played the sport before, struggled to cope with the stamina required.
"He would need a significant amount of additional time to prove whether he would be capable of making the grade," read an official Castleford statement.
"Time is something we don't have, but we wish him the best for the future."
The club said they had concluded the trial with "some reluctance".
Chambers' future is unclear, but Castleford's decision may force him to fight the British Olympic Association's ban on him competing in Beijing later this year.
A BOA by-law prevents athletes found guilty of serious drugs offences from ever appearing at the Olympics, even after they have served their bans.
But Chambers may be able to challenge the BOA's by-law 25 in court in an effort to make the Great Britain team.
Chambers started a trial with the Tigers at the end of March, despite having no previous experience of rugby league.
Both player and club insisted the move was more than just a publicity stunt, but he made only one appearance for the Tigers' reserves during his month-long stay at The Jungle.
He came off the bench to play on the wing and at full-back in a friendly against York City Knights on 27 April but found the going tough and was forced off with a shoulder injury.
Castleford chief executive Richard Wright added: "Dwain has worked very hard during training and the trial game and he showed the commitment and courage to suggest that with time he could become a successful rugby league player.
"However time is not available to us and we have to focus upon more immediate matters and our attention has to be on this year's Super League competition.
"Many people watched Dwain in the trial game and he has certainly won over some of the doubters, and proved that the trial was more than simply a publicity stunt.
"During his trial period Dwain has been a joy to have around. His work ethic and professionalism have been exemplary."
Chambers, who came back from his drugs ban to win a silver medal in the 60m at the World Indoor Championships earlier this year, maintained that he was serious about a career in the sport.
His manager Nick Collins said: "Whilst disappointed, Dwain is comfortable with and understands the decision. He has had a great time and made some good friends during his brief period with the club.
"He leaves the club in a positive mood and with a smile on his face."
Chambers twice failed to break through into American Football.