Celtic midfielder Roy Keane has retired from professional football because of a long-standing hip injury.
Roy Keane's testimonial was his last playing appearance
Keane only signed for Celtic from Manchester United in December 2005 but has decided to quit after receiving medical advice on his hip problem.
The 34-year-old said: "Having received advice from my surgeon and the Celtic doctor, my only option is to retire."
The Republic of Ireland star enjoyed a stellar career at United, winning seven league titles and four FA Cups.
He also won a Scottish Premier League winner's medal during his short spell in Scotland, where he only played 13 games.
"I would like to send the manager, staff, the players and the supporters of Celtic Football Club best wishes for the future," Keane added in a statement.
When they start picking the best teams of all time, he will be in there
Manchester United manager Sir Alex Ferguson
Keane joined United from Nottingham Forest for a then British record £3.75m in 1993 but had a long-standing desire to play for Celtic.
He returned to Old Trafford last month for what was to prove to be his last appearance - in his testimonial between Celtic and United, watched by almost 70,000 fans.
Celtic manager Gordon Strachan said: "Roy Keane is one of the greatest players to grace the game of football.
"It was fantastic that we were able to bring him to Celtic and it has been a privilege to work with him.
"Even in his short time with the club, Roy made a great contribution and played an important part in bringing success to the club last season.
"We were delighted to make Roy's dream come true when we signed him for Celtic and we were happy when he made our dream come true by helping us to win the title.
"Roy's retirement will be a loss to Celtic and, of course, a loss to football in general.
"We would all like to wish Roy and his family all the best for the future. Roy will always be welcome at Celtic Park."
Manchester United manager Sir Alex Ferguson, who made Keane captain at Old Trafford, paid his own glowing tribute to the Red Devils great.
"Roy's obsession with winning and the demands he put on others made him the most influential player in the dressing room," said Ferguson.
"He became a great captain through that and, to my mind, he is the best player I have had in all of my time here.
ROY KEANE FACTFILE
Born: Cork, 10/8/1971
Forest: 154 games & 33 goals
Man Utd: 480 games & 51 goals
Celtic: 13 games & one goal
Ireland: 66 caps & nine goals
Honours: Seven English league titles, four FA Cups, one Scottish title, PFA and Football Writers' Association player of the year (2000)
"Over the years when they start picking the best teams of all time, he will be in there."
A Champions League winner's medal was what Keane craved most at United, but he fell agonisingly short of European club football's major honour.
He led United to a stunning semi-final victory in Turin over Juventus in 1999 but was booked and banned for the final, which United won 2-1 against Bayern Munich.
Keane started his career with Irish outfit Cobh Ramblers before a £10,000 move to Forest under Brian Clough in 1990.
He helped Forest reach the FA Cup final in 1991 but left when the City Ground side were relegated in 1993.
He made his Republic of Ireland debut in 1991 against Chile and went on to win 66 caps for his country, appearing in the 1994 World Cup finals in USA.
He caused a sensation when he left the Ireland squad prior to the 2002 tournament after a huge bust-up with then manager Mick McCarthy.
Never far from controversy, Keane was sent off 11 times during his United career, including a horror tackle on Manchester City's Alf Inge Haaland in April 2001.
Keane later admitted in his autobiography that he deliberately tried to injure Haaland and was banned for five games and fined £150,000 by the Football Association.