The Court of Arbitration for Sport has rejected an International Olympic Committee doping eligibility rule, clearing the way for dozens of athletes who used illegal drugs to compete at next year's London Olympics.

A three-man panel at the Lausanne-based court Thursday upheld the appeal by reigning 400-meter Olympic champion LeShawn Merritt of the United States, deciding the IOC's controversial rule was “invalid and unenforceable.”

The rule banned athletes suspended for six months or more for doping violations from competing at the next Olympics. But the CAS panel said the rule did not conform to the World Anti-Doping Agency Code signed by the IOC, and it allowed athletes to be punished twice.

The 25-year-old Merritt had served a 21-month ban for using a male enhancement product containing a banned substance. His suspension had ended and he returned to competition in July. Now he will be allowed to compete in the 2012 Olympics.

IOC president Jacques Rogge said he was disappointed by the CAS ruling but would fully respect the decision. However, Rogge said he would push for the rule to be reinstated when the World Anti-Doping Agency Code is revised in 2013.

Merritt said he is looking forward to defending the gold medal he won at the 2008 Beijing Summer Games and representing his country, and “will prepare with even more determination than before.”

Meanwhile, the British Olympic Committee (BOA) said it is committed to defending its own rule of lifetime bans for any of its athletes previously banned for doping from competing next year in London.

Two top Britons that could be impacted are cyclist David Millar and sprinter Dwain Chambers, who have both served two-year doping bans.

The BOA said it will be writing to the IOC for extra support despite Thursday's court ruling in Lausanne.