The ruling by the Court of Arbitration for Sport, has raised fundamental doubts about the British Olympic Association’s life ban on doping offenders.

The CAS ruling in the case of US sprinter LaShawn Merritt has already cleared the way for hundreds of convicted drugs cheats to compete in London.

The International Association of Athletics Federations estimates that more than 50 track-and-field athletes can now be selected, and some 33 athletes banned by the US Anti-Doping Agency are also affected.

Olympics sources said that the total number of athletes affected was likely to run well into three figures.

Under the BOA by-law, any athlete banned for more than six months for doping is banned for life from Team GB, a rule that catches Millar, who was banned for two years in 2004 after admitting using EPO, and sprinter Dwain Chambers.

On Thursday Millar said he was confident he could overturn the BOA by-law but had “no idea” if he would launch a challenge as his priority was to respect the British cycling team.

“I have no idea what I am going to do, I am going to let the gods decide,” he said.

“If I appealed, I believe I would have a high chance it would succeed. But I have a lot of respect for Dave Brailsford and the Great Britain squad and I don’t want to cause any grief or negative press.”

Millar’s commitment to anti-doping since his ban has seen him appointed to the WADA Athlete Committee, but he is still cautious about an appeal.

“If I went to court I’m sure I would beat it but I’m also concerned about the haters crawling out of the woodwork, I’ve had enough of that. The only pro would be for me personally to be able to compete in London.”

Chambers, who is understood to be in Jamaica following the death of his father, is still considering his options.

His adviser said the sprinter, who unsuccessfully appealed against the by-law before the 2008 Olympics following a two-year ban in 2004, said he would “digest and consider” the CAS judgment before making a decision.

As well as Chambers, Millar and shot-putter Carl Myerscough, 11 British athletes banned under the UK anti-doping system since 2008 could also consider challenges.

BOA chairman Lord Moynihan said he was ready to defend the by-law, but opinion among experienced lawyers in the anti-doping field, as well as Olympic and anti-doping sources, suggests it is now vulnerable.

Describing the Merritt ruling as “a sorry day for the IOC”, Moynihan said the by-law was “tough but fair” and had the overwhelming support of athletes.

“It pursues a legitimate aim which reflects the Olympic Games as the pinnacle of sporting achievement, embraces the Olympic ideals, and the fundamental principles of clean sport,” he said.

Moynihan said that the BOA had received a letter from WADA confirming that the by-law conforms with the anti-doping code, and said he had written to the IOC requesting an endorsement of its right to select the athletes it chooses.

He also stressed that the BOA’s position differs from the IOC’s now-discredited Rule 45, though that was challenged by lawyers.

CAS declared Rule 45 “invalid and unenforceable” because it went beyond the standard two-year sanctions under the WADA code to which all Olympic sports, and the IOC, are signatories. This means the IOC could not ban athletes who had already served their penalty, such as Merritt, from the next Olympics.

The IOC claimed that Rule 45 was not a sanction, but was based on eligibility to compete in the Olympics, but CAS rejected that argument. The BOA is also arguing that its by-law is about eligibility rather than a “sanction”.

Millar said he believes the ban is unsustainable.

“It is only a matter of time before it is annulled. I can understand where the by-law comes from and I can see where the origins lay, but the bottom line is that everything has changed since then.

"It doesn’t make sense to have a universal draconian penalty that doesn’t distinguish between offences.”