LONDON (AFP) - British advertising regulators on Wednesday ordered Irish bookmaker Paddy Power to immediately withdraw a campaign offering "money back if he walks" about the murder trial of South African athlete Oscar Pistorius.

The Advertising Standards Authority (ASA) said it had received more than 5,200 complaints about the ad - an "unprecedented number" - after it appeared in the British and Irish press.

More than 100,000 people had also signed a petition demanding Paddy Power pull the advert and close its book on the trial of Pistorius, who is accused of killing his girlfriend, Reeva Steenkamp, on Valentine's Day last year.

"We consider the ad may be seriously prejudicial to the general public on the ground of the likely further serious and/or widespread offence it may cause," the ASA said in a statement.

"We are also concerned that the good reputation of the advertising industry may be further damaged by continued publication of this ad." It added: "We are investigating whether the ad is offensive for trivialising the issues surrounding a murder trial, the death of a woman and disability."

The advert depicted an Oscar statuette with Pistorius's face and the words: "It's Oscar Time. Money Back If He Walks. We will refund all losing bets on the Oscar Pistorius trial if he is found not guilty."

No stranger to publicity stunts, Paddy Power timed it to coincide with Sunday's Academy Awards and the opening of the trial in Pretoria on Monday.

The ASA has taken the unusual step of ordering Paddy Power to withdraw the advert in Britain pending the outcome of an investigation by the watchdog's council.

Paddy Power was not immediately available for comment, but the ASA said the bookmaker had expressed a "willingness to cooperate" with the investigation.