The Court of Arbitration in Sport, based in Lausanne, France, ruled the lifetime competition ban given Chambers after he failed a 2003 performance-enhancing drug test given by the British Olympic Association, the national Olympic committee of Great Britain and Northern Ireland, did not comply with the code of the World Anti-Doping Agency and is consequently unenforceable.
As a result, the British association is powerless to prevent Chambers from competing in July's Summer Games in London, if he is selected to represent Great Britain.
Millar, who was banned from eligibility under similar circumstances, can also compete for a position on the British team, the London newspaper The Daily Telegraph reported.
Under the terms of the anti-doping agency's code, which includes the British association as a signatory, doping penalties must be uniformly enforced around the world. The sports court ruled against the British association on the grounds the lifetime bans handed Chambers and Millar were additional penalties imposed on top of two-year suspensions they were given earlier.
Millar was excluded from cycling in 2004 after he admitted using the outlawed drug EPO following a French police investigation.