Manchester United manager Sir Alex Ferguson has little doubt that he will step down in three years' time.

By then Britain's most decorated boss will have spent 25 years at the helm at Old Trafford.

Ferguson has won 18 trophies after arriving at the club in November 1986 from Aberdeen - and remains anxious for more.

United are still in line for the treble for the first time since 1999 and face Portsmouth in the FA Cup on Saturday.

They are also aiming to retain their league title and have already booked a place in the quarter-finals of the Champions League this week after beating Lyon.

Ferguson had initially planned to retire in 2002, only to change his mind.

That decision was vindicated as he rebuilt his side, going on to claim last season's Barclays Premier League crown.

Ferguson believes it will be time go when he is 69.

"I'm getting beyond the future now," he told French radio.

"I still have a lot of passion. I'm still happy. But I'm 66 now - maybe three years more, then I'll finish."

Ferguson said it was too soon to mark out his successor. But he added: "I have a great assistant in Carlos Queiroz. He will come into the reckoning.

"But we have owners and a chief executive who will choose the new man in three years' time.

"The thing about football is we have some potential young managers but, because of the dangers, the hazards of management, they may not be there in three years' time.

"The change in football is so much so quickly. If you're not successful in three games you could be away. All these young managers we've seen over the years are not there now."

Ferguson's retirement is a prickly issue at Old Trafford - not least in the manager's office. When the matter was raised at a press conference on the eve of his 20th anniversairy in charge of the club, an enraged Ferguson responded: "It is scandalous some people think I should retire. It is none of their business. Some people in this country don't want to work, so I don't think you should decry anyone who wants to.

"It disgusts me that people think that way. It should not be allowed. Legislation on retirement is changing in this country, which is right. There are no retirement age laws in the USA."

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