Olympic medallist and ex-world champion Rob Hayles has been suspended for two weeks after a blood test at the Track Cycling World Championships.
Hayles, 35, who won two silvers at the 2004 Olympics, was set to race for Britain in Manchester.
His blood result was 0.3% over the permitted 50% haematocrit level.
The result is an indication, but not proof, that illegal blood-boosting may have taken place but British Cycling says it can also occur naturally.
On learning of the result, Hayles said: "I am disappointed and frustrated to have to wait for further tests over the next two weeks before I can compete again. I accept the process is in place for good reason and I agree with it.
606: DEBATE
"I also accept that I must follow protocol. These anomalies have happened to others across the world, and after two weeks they have been resolved.
"So I just have to wait for two weeks."
Haematocrit level is the percentage of red blood cells in a person's blood.
Dutch rider Pim Ligthart has also been suspended for 14 days after a blood test.
A total of 66 tests were carried out on riders from the United States, Australia, the Netherlands and Britain on Wednesday morning. All the other riders were cleared to race.
"I've known Rob a long time and there's never been any doubt in my mind that he's been anything but a fantastic athlete - he does not take drugs"
The British rider was due to race in the individual pursuit event on Wednesday, the opening day of the Championships. Ligthart was entered for Friday's men's points race.
Dave Brailsford, British Cycling performance director, told BBC Radio 5 Live: "I am sure there is an honest explanation.
"I am astonished and disappointed for Rob and frustrated for the team.
"But at the end of the day I think the truth will prevail as always. I welcome the screening system - I think it is a great thing. I think we should be subject to it and I would like to see other nations subject to it.
"This is not the first time this has happened to several riders and you tend to find that there are subsequent tests that are then actually verified by the UCI.
"I have known Rob a long time and there has never been any doubt in my mind that he has been anything but a fantastic athlete for Britain. He does not take drugs."
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