Australian rugby league great Andrew Johns says he has been a fool and getting caught with an ecstasy tablet in London is a black mark on his reputation.

The retired Newcastle, NSW and Kangaroos star, regarded by many as rugby league's greatest player, was issued with an official caution for possession of an ecstasy tablet in London on Sunday.

In a statement, he said the ecstasy tablet was pushed into a pocket of his jeans at a crowded club by an "unknown" person.

Johns, 33, was not charged and won't have a conviction against his name, but NSW Premier Morris Iemma said if caught with the drug in NSW he could have been prosecuted.

The Australian Rugby Union (ARU) had been considering using Johns to help the Wallabies in their build-up to next month's Rugby World Cup in France, but promptly dropped the idea.

Johns was arrested by transport police conducting routine searches at Kings Cross underground railway station following the annual Notting Hill Carnival, a statement from Johns' management The Fordham Company said.

"Johns said he recalled an unknown person pushing a tablet into his jeans at a crowded club on Sunday - his last day in the United Kingdom following a six weeks overseas holiday."

In the statement, Johns said: "I was having such a good time, seeing it was the last day before I flew home.

"I stupidly forget about the tablet and instead of getting rid of it I left myself in a situation I soon deeply regretted."

NRL chief executive David Gallop was disappointed by the affair and Johns' excuse.

"Andrew's not a registered player in our competition so he's not the subject of our penalty regime but clearly he's still a very important role model and therefore it's very disappointing," he said.

"That explanation's is clearly going to come under some scrutiny, but I'll leave it to him to talk more about that.

"These types of situations are damaging to the image of the game and damaging to his reputation, there's no doubt about that."

Johns is regarded as a role model in the sport and he acknowledged his actions would disappoint many young fans.

"I have been a very foolish person and realise I have brought great disappointment to my family, friends and many people, including lots of youngsters who have looked up to me over the years," he said.

"It has left a black mark on my reputation which I will work very hard at erasing."

Johns' manager John Fordham said news of the bust had been "devastating" for the former player.

"But he's working hard at getting control of all of that - they're difficult times," he told Nine Network.

Mr Fordham admitted Johns was lucky to have escaped with just a caution.

Mr Iemma, an NRL fan, reminded Johns his status bore certain responsibilities.

"Footballers are role models, but in relation to drugs and in particular ecstasy, they are dangerous," the premier said. "In this state it could well have led to charges."

In Britain a formal caution does not mean a criminal conviction.

However it is recorded on the police database and may be considered in court if a person is tried for another offence, the UK Home Office says.

Johns, who was forced out of the game in April by a recurring neck injury, has been holidaying in Europe and the UK.

He had been in London for a few weeks, helping to coach the Harlequins Rugby League Club at the invitation of former Newcastle Knights player Billy Peden.

Johns recently spent time tutoring the Wallabies' kickers and coach John Connolly suggested the former halfback may work with the team again in France, once his NRL commentary commitments were completed.

But ARU boss John O'Neill said there had been no firm deal with Johns.

He added: "Given the current circumstances, the Wallabies will continue as is in France."

Wests Tigers NRL coach Tim Sheens said players were human and did silly things, but most did the right thing.

"We can only keep battling to maintain an image and do the right thing and I think there's been so much work done in that regard it's a shame to see one particular tablet in someone's pocket cause so much disturbance."

Johns was due to appear on Nine's The Footy Show tonight.

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