Turnbull is being backed by his training partner Mark Carroll

Turnbull's testosterone ratio in the drugs test from last September is known to have been between 5:1 and 6:1.

However, prior to 2005 readings of 6:1 and under were permissible under then World Anti-Doping Agency (Wada) rules.

But in 2005, a Wada rule-change reduced the highest legal testosterone ratio from 6:1 to 4:1.

Explaining the change at the time, Wada said:"The probability of occurence of a naturally-occurring ratio above 4:1 is extremely low.

"Furthermore a ratio of 4:1 or above will not necessarily lead to a sanction but will require that the anti-doping organisation conducts further investigations to determine whether the ratio is the result of anabolic steroids intake or due to the athlete's physiological condition."

Gareth is such a solid man and full of integrity Mark Carroll

Meanwhile, Turnbull's regular training partner Mark Carroll has told the BBC that he is "certain" the Belfast athlete is not guilty of a doping offence.

"He didn't take anything and would never take anything that would yield a positive result," said Carroll.

Carroll, European Indoor 3000m champion in 2000, added that the doping charge levelled against Turnbull was "crazy".

"There's got to be something wrong or a reasonable explanation," said Carroll.

"Gareth is such a solid man and full of integrity.

"He's completely passionate about the sport and he is totally in favour of anti-doping procedures."

Carroll added that he was optimistic that a more detailed carbon isotope examination of the urine sample would clear Turnbull's name "relatively quickly".

"The carbon isotope test works in that if you had administered testosterone by injection or orally, carbon metabolites would have to be present in the urine.

Carroll is certain that Turnbull has not committed an offence

"This test proves absolutely whether a steroid has been administered but I don't know whether that test has been carried out."

Irish 5000m record-holder Carroll is well-known for being a vociferous opponent of drug-taking in his sport.

Over the last number of years, Turnbull has spent much of his winters and springs at Carroll's US base in Florida.

Turnbull is understood to have had several tests before and after the 30 September test which showed normal testosterone levels.

The runner is certain to ask the Irish Sports Council to test the B-sample of the urine that he submitted on 30 September.

However, the Belfast athlete has been provisionally suspended and the timing of the shock news means his hopes of competing in this year's European Championship are now in doubt.

Turnbull's case will now be referred by the Irish Sports Council to a three-person disciplinary panel who will make a judgement.

Guilty verdicts in these kind of doping cases usually mean a two-year ban although there would be the option of making an appeal to the Court of Arbitration for Sport in Switzerland.