Former Olympic Australian swimmer Grant Hackett has apologised after he was accused of groping a fellow passenger during a flight.

The 35-year-old, who has three Olympic golds to his name, is alleged to have assaulted a fellow passenger when the man reclined his seat.

The individual told the Herald Sun that Hackett "yanked the seat back" and then "he groped my chest and tweaked my nipple quite forcefully."

In response to the allegations, Hackett released a statement, which read: "I have stuffed up more than once and am working on these issues.

"I seriously and genuinely regret my poor behaviour. It is embarrassing to hear and read the consequences of my actions. I apologise unreservedly to the gentleman on the flight. I am trying to make direct contact with him."

Hackett recently came out of retirement from professional swimming in a bid to qualify for the Rio Games, but did not manage to earn a spot.