Alex Ferguson will retire as Manchester United manager at the end of this season, the English Premier League champions announced Wednesday, bringing down the curtain on one of the most successful careers football has known.

Ferguson, 71, has been in charge at Old Trafford for 26 years, guiding United to 13 Premier League titles and two Champions League crowns.

It was not until Tuesday that rumours of Ferguson's shock retirement started circulating but he said it was a decision he had been considering for some time.

"The decision to retire is one that I have thought a great deal about and one that I have not taken lightly. It is the right time," Ferguson said in a statement released by the club.

"It was important to me to leave an organisation in the strongest possible shape and I believe I have done so," added Ferguson, who will bow out with United having won this season's Premier League.

"The quality of this league-winning squad, and the balance of ages within it, bodes well for continued success at the highest level whilst the structure of the youth set-up will ensure that the long term future of the club remains a bright one."

United gave no indication of a successor amid speculation Everton's David Moyes could fill the Old Trafford hot seat vacated by fellow Scotsman Ferguson.

Real Madrid manager Jose Mourinho, who established a friendly rivalry with Ferguson when in charge of United's rivals Chelsea, has also been touted for the role.

David Gill, who forged a highly succesful partnership with Ferguson as United's chief executive that included the unprecedented Treble of 1999 which included the Premier League, Champions League and FA Cup, paid tribute to the veteran manager.

"I've had the tremendous pleasure of working very closely with Alex for 16 unforgettable years - through the Treble, the double, countless trophy wins and numerous signings.

"Alex's vision, energy and ability have built teams - both on and off the pitch - that his successor can count on as among the best and most loyal in world sport."

United said Ferguson would remain at Old Trafford as a director and club ambassador. His final game in charge will be against West Bromwich Albion on May 19.

Ferguson is due to have a hip replacement operation after the end of the season but until rumours broke Tuesday there was no indication he was about to step down.

The former Glasgow Rangers' striker's managerial career began with Scottish lower league club East Stirlingshire in 1974 and saw him win 49 major trophies in all.

Ferguson forged his managerial reputation in charge of Aberdeen, taking the Dons to a trio of Scottish Premier League titles and the 1983 UEFA Cup' Winners Cup before joining United in 1986. (AFP)