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The former rugby league star Andrew Johns has admitted that he was arrested for possession of an ecstasy tablet in London, but has blamed an "unknown person" for pushing it in his pocket at a notorious inner-city bar.

Johns has nonetheless described himself as a "very foolish person" and vowed to repair the damage done to his reputation.

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

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

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

Johns was arrested and cautioned for possession of an ecstasy tablet at London's Kings Cross station.

British transport police confirmed that Johns, who was believed to have been returning from the Church nightclub in nearby Kentish Town, was arrested at 4.20pm on Sunday.

The Church is a well-known venue for Australian expats and tourists. Willie Mason and Anthony Minichiello were famously pictured there in 2004.

By accepting an official warning, Johns has in effect admitted to the offence and now has a criminal record in Britain.

Johns was caught in a routine search by local police, who were targeting revellers returning from the annual Notting Hill Carnival in west London.

A transport police spokesman told the Sydney Morning Herald last night that "an Australian man in his 30s was arrested and cautioned for possession of an ecstasy tablet".

Johns will not be charged over the offence and is free to return to Australia. Another similar caution, however, would result in a charge and a possible court appearance, the transport police official confirmed.

The former Australian Test captain had been holidaying in Britain, and had spent the previous week coaching the rugby union club Harlequins and attending the Challenge Cup final at Wembley Stadium.

Simon Moran, the majority shareholder of the Warrington Wolves, had been with Johns at the Challenge Cup final last Saturday.

"He seemed fine, no problem at all," Moran said. "He had previously been up and worked with two of our halfbacks and the juniors for a day.

"I'm pretty sure he's gone home now. I have no other comment to make."

Johns retired from rugby league earlier this season, ending one of the most celebrated careers in league history.

The Newcastle star had received a succession of head knocks, and medical staff advised him to retire for fear of permanent spinal injury.

More recently, Johns has worked as a kicking coach in a consultancy capacity with the Knights and Harlequins.

Johns also spent time tutoring the Wallabies' kickers before the Australian side upset the All Blacks in Melbourne in June.

However, following Johns's caution, Australian Rugby Union boss John O'Neill ruled out any chance of the former Kangaroos captain joining the Wallabies as a coaching consultant during the World Cup in France.

Australian coach John Connolly said recently he hoped Johns could work again with the team in France once the former halfback's commentary commitments with the Nine Network were completed after the NRL finals series.

But O'Neill said today the ARU had not entered into any formal arrangement with Johns.

"It wasn't a done deal that he'd be contributing to the team in France," O'Neill said through a spokesman today.

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

NSW Premier Morris Iemma reminded Johns that footballers are role models and warned him he could have faced criminal charges if he had been caught in NSW.

"I note that he's made an apology. Footballers are role models, but in relation to drugs and in particular ecstasy, they are dangerous," he said.

"In this state it could well have led to charges."

- with AAP