American LaShawn Merritt, who served a 21-month drug ban, is shown after winning the men’s 200 meters at an IAAF Grand Prix meet in Italy in September. (Riccardo De Luca/AP)

An International Olympic Committee rule that bars Olympic 400-meter champion LaShawn Merritt and other athletes from participating in the 2012 Summer Games in London is “invalid and unenforceable,” according to a decision announced Thursday morning by an independent Swiss court.

The decision by the Court of Arbitration for Sport supported the U.S. Olympic Committee’s challenge of a three-year-old International Olympic Committee rule that bars athletes who have served drug bans of six months or longer from competing in the next Olympic Games.

A three-judge panel concluded in a 23-page decision that Rule 45 of the Olympic Charter, enacted in July, 2008, is “not in compliance with [world anti-doping rules] and violates the IOC’s own Statutes.”

Merritt’s status for the 2012 Summer Games in London has been in question since he tested positive three times for a steroid in a male enhancement product in 2009-10. He served a 21-month ban that ended this summer. He is the most prominent U.S. or international athlete whose Olympic hopes were jeopardized by expired drug bans.

Merritt’s case prompted the USOC and IOC to file the request for joint arbitration over a rule that proved divisive as soon as it was enacted. The IOC said Thursday it would abide by the panel’s decision, but expressed surprise and disappointment, arguing that the rule had offered an “efficient means” of advancing the fight against doping.

“The IOC has a zero tolerance against doping and has shown and continues to show its determination to catch cheats,” the organization said in the statement. “We are therefore naturally disappointed since the measure was originally adopted to support the values that underpin the Olympic Movement and to protect the huge majority of athletes who compete fairly. ”

Merritt was scheduled to comment at a press conference in Atlanta Thursday afternoon. His attorney, Howard Jacobs, hailed the ruling in a phone interview, calling it the “correct decision,” and saying “it was made for the right reasons.”

The ruling stated that if the IOC wanted to bar athletes who had served drug bans from the Olympics, it should propose an amendment to the World Anti-Doping Code. The Code, which governs anti-doping in the Olympic movement, expressly prohibits additional penalities for athletes whose bans have ended.

In its statement, the IOC said it would ensure that its rule, and other tougher sanctions, would be considered during the next revision of the Code.

During an Aug. 17 hearing, the IOC contended that its rule represented a legitimate eligibility restriction, not an additional sanction, but the CAS panel disagreed.

Anti-doping officials, including Travis Tygart, the chief executive of the U.S. Anti-Doping Agency, had publicly argued that the IOC’s rule amounted to double jeopardy, forcing athletes to pay twice for one crime.

“Like the IOC, we are in full support of clean competition and stringent anti-doping penalties,” USOC Chief Executive Officer Scott Blackman said in a statement. “This decision does not diminish our commitment to the fight against doping, but rather ensures that athletes and National Olympic Committees have certainty as they prepare for London.”

The British Olympic Association places its own lifetime Olympic bans on British athletes who have tested positive; that prohibition kept sprinter Dwain Chambers off the Olympic team in 2008.

Canadian attorney Richard McLaren, American David W. Rivkin and Swiss lawyer Michele Bernasconi composed the panel.

Merritt, a native of Portsmouth, Va., who attended Old Dominion and Norfolk State, produced a stirring upset victory over then-reigning champion Jeremy Wariner at the 2008 Summer Games, then followed that up by winning the world title in Berlin in 2009.

The U.S. arbitration panel that handed down Merritt’s ban ruled in October 2010 that he did not intend to cheat and likely did not enhance his performance on the track when he used an over-the-counter product called ExtenZe several times during the winter of 2009-10.

The product contains the steroid DHEA, which has long been on anti-doping banned lists but is not considered a powerful steroid. Even so, the fact that the steroid was in his system mandated the extensive ban.

Though Merritt did not return in time to compete at the U.S. championships in Eugene, Ore., in June, he petitioned USA Track and Field for a bye into the world championships in Daegu, South Korea. There, he won a silver medal in the 400.