Johns was regarded as the best player in the world in his prime

The scrum-half great, 32, was told he risked a "catastrophic" spinal injury by playing on after undergoing CAT scans and seeing specialist John Yeo.

He aggravated a previous neck injury in a collision with team-mates in training with his club Newcastle last Thursday.

"I'm still in shock. It was really tough telling my family," Johns said.

"I made an emotional call to my brother Matthew and it was even tougher talking to the (Newcastle) players this afternoon."

This could have happened when I was a 19-year-old Andrew Johns

Johns decided to end his 15-year career after Knights club doctor Neil Halpin said the scans revealed a previously undetected disc injury which, while free of painful symptoms, posed a serious risk.

"Talking to Neil and the professor (Yeo) I realised how lucky I am that I haven't had a serious neck injury or a serious accident and could be in a wheelchair," Johns added.

"Because it only had to be one knock and it was a fairly simple knock that I got at training and I was in agony, I would hate to think what a full blown hit in the field could have done."

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Johns, regarded as possibly the most gifted scrum-half in rugby league history, led the Newcastle Knights to the National Rugby League Premiership title in 1997 and 2001.

He played 23 State of Origin games for New South Wales and 21 Tests for Australia, playing in their 1995 and 2000 World Cup-winning teams.

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Johns also notched up 2,176 points from his 249 first-grade games for Newcastle.

Johns intended to retire at the end of the season anyway, but his premature retirement is also a blow to British fans.

Johns had agreed to play as a guest for New Zealand in the Centenary International at Warrington in November to honour the contribution of Dally Messenger, the Australian who played for the Kiwis on their first British tour back in 1907.

"I would have loved to have finished with a Premiership but this could have happened when I was 19-year-old," he added.

"I've achieved so much in the game and I regard myself to be very lucky.

"It is a shock, but I'm not totally disappointed because I've had some great memories and some great friendships which is the most treasured thing I've got out of the game.

"I've had a fairytale career and I feel lucky to have played the game."

Australia Rugby League chairman Colin Love said Johns, known as "Joey", was one of the greatest players to have played the game.

NRL chief executive David Gallop added: "For 15 years he has been a colossal figure in our game."