"I don't have any proof, but it's my gut feeling doping exists," Hall said as he met the press on the first day of the eight-day US Olympic swimming trials.

"Do I think it is getting worse?" Hall said. "Yes, I do. It's here, it's in the United States.

"I train with an international group of swimmers, and all of them have stories and a few of them have had offers and I'm not at liberty to say," added the 33-year-old.

"Unfortunately we rely on inadequate doping agencies for proof."

Hall is a four-time Olympic gold medallist and the two-time defending champion in the 50m freestyle.

This week he will try to qualify for a chance to defend that title in Beijing. No male swimmer has ever won the same individual swimming event three times.

Hall said that in his opinion drugs have played as much of a role as the new high-tech swimsuits - particularly Speedo's LZR Racer - in a spate of world records in the first half of the Olympic year.

Of 20 long-course world records set this year, 19 have been by swimmers wearing the LZR, which was introduced in February.

"I am convinced there is an advantage to wearing the suits, but I don't think it accounts for all the time drops we've seen," Hall said.

Hall said the biggest danger in the fight against doping was complacency, not only on the part of officials and competitors but also from fans, who are wearied by the scandals that have tainted athletics, cycling and baseball.

"What bothers me most is the resignation and the attitude of the public that it is just part of sport today," he said. "We need to get over that, because it is not OK."

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