Here are some of the leading candidates for arguably the biggest job in world soccer:

Jose Mourinho

The charismatic Portuguese introduced himself to Manchester United with his famous jig down the touchline when his Porto side knocked United out of the Champions League in 2004 on the way to winning the tournament.

His work at Chelsea, where he won two league titles, backed up Mourinho's claim to be "the special one" and since leaving Stamford Bridge in 2007 he has won league titles in Italy (Inter Milan) and Spain (Real Madrid).

He also claimed the European Cup for Inter and is desperate to achieve the feat in a third country.

While Mourinho is being tipped for a return to Chelsea next season, a call from United could prove irresistible for a coach with Mourinho's ego and who has enjoyed a friendly rapport with Ferguson down the years.

However, Mourinho's love of the limelight and his outspoken nature, which often gets him into hot water, might not sit too comfortably with the United 'brand' while his preference for tactical pragmatism over flamboyance would be a concern for fans brought up on United's free-flowing football.

David Moyes

The third-longest serving manager in England's top flight after Ferguson and Arsene Wenger, Moyes has a shrewd tactical ability and work ethic which have enabled Everton to compete towards the upper reaches of the league despite a relatively small budget.

Like Ferguson, Moyes was born in Glasgow and he appears to be cut from the same cloth. A ferocious stare demands respect from his players and, while clearly a strong leader, Moyes has also proved himself adaptable.

He took Everton into the top four in 2004-05 but lost in the final qualifying round of the Champions League.

While never having won any silverware for Everton, Moyes has overseen consistent improvement and they challenged for the top four again this season.

Personality-wise Moyes would be a natural replacement for Ferguson but the downside is that he has never endured the kind of pressure associated with being in the hot seat at one of the world's biggest clubs.

Juergen Klopp

The 45-year-old coach of Borussia Dortmund has proved himself in the Bundesliga since taking over in 2008.

He steered Dortmund to a first domestic double last year and although Bayern Munich have won the title this season by a large distance, his stock has risen further after the club's win over Real Madrid in the Champions League semi-finals.

Klopp recently said he wanted Dortmund to "play football that people remember" and that philosophy would sit well with the United hierarchy.

Popular with the players, Klopp is also charming with the media and if United are to go down a foreign route they may see him as a less "high-maintenance" choice than Mourinho whose relationships with both can be strained at times.

Working against him could be his lack of intimate knowledge of the Premier League.

Ole Gunnar Solskjaer

Known as the "baby-faced" assassin during his playing days at United, the highlight of which was his dramatic late winner in the 1999 Champions League final, the Norwegian has been well-tutored in Ferguson's management skills having coached the reserves after hanging up his boots.

Now at his boyhood club Molde whom he took to consecutive Norwegian titles, the 40-year-old Solskjaer is still highly-regarded at United with Ferguson last year tipping him as his eventual successor.

"Ole was always one of the professionals that used to take down all the notes from the training sessions and games," Ferguson said last year. "He has got an inner toughness, there's no doubt about that."

Michael Laudrup

An outsider, maybe, but the Dane is turning heads with his work at Swansea City, leading the Welsh club into Europe by winning the League Cup this season, their first major trophy.

One of the most skilled players of his generation, Laudrup preaches attractive passing football and his Swansea side have been one of the most pleasing on the eye this season, securing their top-flight survival with ease.

Before succeeding Brendan Rodgers at Swansea he briefly enabled Mallorca to punch well above their weight in Spain with a free-flowing style.

Laudrup topped a recent fans' poll at Real Madrid on who should follow Mourinho, should he leave, but the challenge of following Ferguson would appeal to the ambitious coach.

Ryan Giggs/Gary Neville

Welsh winger Giggs was discovered by Ferguson as a teenager and has gone on to become one of the club's greats with more than 900 appearances and counting.

Giggs is already preparing himself for life after playing by taking his coaching badges, but it is highly unlikely that he would be thrust into the top job so soon.

However, with his old team mate through the youth ranks to the first team Neville, also apparently cut out for coaching, they could form a future "dream team" alongside an experienced older coach such as Moyes or Martin O'Neill