By Mitch Phillips

RIO DE JANEIRO (Reuters) - The head of British anti-doping said on Tuesday she was still waiting to hear from the Court of Arbitration for Sport (CAS) for a full explanation of why it overturned a ban for road cyclist Lizzie Armitstead for missing three doping tests.

The decision was also questioned by fellow cyclists, with one of the Briton's main rivals in Sunday's Olympic road race describing it as "just shameful".

The 2015 world champion and 2012 Olympic silver medalist was facing an automatic ban under the "whereabouts system" after three missed tests within a year but CAS upheld her appeal that correct procedures were not followed for the first of the tests, allowing her to compete in Rio.

Nicole Sapstead, chief executive of UK Anti-Doping (UKAD) issued a statement on Tuesday questioning why Armitstead did not challenge the initial missed test finding last August, choosing to do so only last month after two subsequent missed tests triggered the ban last month, which was kept secret.

"When UKAD asserts a Whereabouts Failure against an athlete, the athlete has the opportunity to challenge the apparent Whereabouts Failure through an external Administrative Review, before it is confirmed," Sapstead said.

"Only when three Whereabouts Failures are confirmed is the case then put through an independent review to determine whether the athlete has a case to answer.

"Ms Armitstead chose not to challenge the first and second Whereabouts Failures at the time they were asserted against her. At the CAS hearing, Ms Armitstead raised a defense in relation to the first Whereabouts Failure, which was accepted by the Panel. We are awaiting the Reasoned Decision from the CAS Panel as to why the first Whereabouts Failure was not upheld."

Under the "Whereabouts System" athletes are required to inform anti-doping officials of a location where they can be contacted during a particular hour of every day.

British former 400m Olympic and world champion Christine Ohuruogu served a one-year ban in 2006 after missing three tests, as did French former triple jump world champion Teddy Tamgho in 2014.

Armitstead issued a statement on Monday saying that the UKAD official trying to locate her in Sweden had not been given her hotel room number and had then tried to contact her on her mobile, which was switched off.

CAS ruled that the tester had not tried hard enough and said Armitstead had not been negligent.

She did not challenge the other two failures, which she said were due to administrative failures on her part.

Some other Olympic athletes have voiced concerns over the decision, and questioned why British Cycling funded and backed her appeal.

France's 2014 road race world champion Pauline Ferrand-Prevot tweeted "Juste honteux" (just shameful).

Canadian Three-times Olympian mountain biker Geoff Kabush tweeted: "So many questions. How is World Champ suspended for three weeks and no one knows? Why did British Cycling fund appeal?

"First test understandable but I'd be hyper aware about missing second. If I missed second there is no chance I'd miss third???"

British Olympic rowing gold medalist Zac Purchase said: "Given huge amount of resources at their disposal, having multiple missed tests/filing failure is a monumental cockup.

"Imagine what we would be saying if she was Russian..."

In an interview with Britain's Daily Telegraph earlier this year Armitstead said she wanted to see lifetime bans for anyone who failed a doping test and had little sympathy for those claiming they had made errors.

"People make mistakes, yeah," she said. "But they're idiots."

(Editing by: Toby Davis)