LONDON (Reuters) - The United States must prosecute former President George W. Bush for torture if his admission in a memoir that he authorized waterboarding holds true, rights group Amnesty International said on Wednesday.

Demonstrator Maboud Ebrahimzadeh is held down during a simulation of waterboarding outside the Justice Departement in Washington November 5, 2007. REUTERS/Kevin Lamarque

In “Decision Points,” published this week, Bush defended his decision to authorize waterboarding, a form of simulated drowning condemned by some as torture.

Bush said the practice was limited to three detainees and led to intelligence breakthroughs that thwarted attacks and saved lives. He told NBC in an interview to publicize the book that his legal adviser had told him it did “not fall within the anti-torture act.”

Amnesty International’s Senior Director Claudio Cordone said in a statement: “Under international law, anyone involved in torture must be brought to justice, and that does not exclude former President George W. Bush.

“If his admission is substantiated, the U.S.A. has the obligation to prosecute him,” he said. “In the absence of a U.S. investigation, other states must step in and carry out such an investigation themselves.”

Waterboarding was banned by Bush’s successor, President Barack Obama, shortly after he took office in 2009.

Interrogators are now required to follow interrogation guidelines laid out in the U.S. Army Field Manual, which excludes the practice.

Bush wrote that waterboarding was first approved for Abu Zubaydah, an al Qaeda figure arrested in Pakistan in 2002 who was suspected of involvement in a plot to attack Los Angeles International Airport.

When Abu Zubaydah stopped answering questions from the FBI, CIA Director George Tenet told Bush he thought the detainee had more information to offer.

Bush wrote that there were two techniques, which he did not describe, that he felt went too far even though they were legal and he ordered that they not be used. But he approved the use of waterboarding.

“No doubt the procedure was tough, but medical experts assured the CIA that it did no lasting harm,” he wrote.