Top secret documents show that the FBI suspected that Muhammad Ali's devastating victory over Sonny Liston 50 years ago was fixed, Britain's Daily Mail reported on its website.

World heavyweight champion Liston - at the time regarded as virtually invincible and known as the ''Big Bear'' - defended his title against 22-year-old Cassius Clay, a 7-1 underdog, at the Miami Convention Centre on February 25 1964.

The youngster used his blisteringly quick hand speed and nifty footwork to give the champ a boxing lesson. Liston was unable to answer the bell at the start of the eighth round.

According to the Daily Mail, documents released to The Washington Times reveal that the FBI suspected the fight had been fixed by Ash Resnick, a Las Vegas gambler with organized crime connections who was connected to Liston.

The documents, released under the Freedom of Information Act, were so sensitive that they were addressed to FBI director J Edgar Hoover personally.

There were suggestions that both Resnick and Liston made over $1-million betting against Liston in the fight.

But, as the Daily Mail reports, the papers show no evidence that Ali was aware of the alleged scheme and "nothing suggests that the bureau ever fully corroborated the suspicions it investigated''.