Sir Alex Ferguson has hinted that he will spend three more years at Old Trafford before he finally decides to step down as manager of Manchester United.

The 66-year-old Scot has won 18 trophies since arriving at the club in November 1986 and is now British football's most successful manager. If Ferguson does decided to leave Manchester United in 2011, he will have completed 25 years as manager and despite still having the drive to succeed he admits he can't go on for ever.

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

Manchester United remain on course for the treble, and face Portsmouth in the FA Cup at Old Trafford tomorrow. They remain just one point behind Arsenal in the race for the Premier League title and booked their place in the quarter-finals of the Champions League this week after beating Lyons 2-1 on aggregate.

Ferguson had planned to retire in 2005, only to change his mind, however his decision was vindicated when his resbuilt side won the Premier League, for the ninth time in his career.

However, the former Aberdeen manager has refused to be drawn on a possible successor.

"I have a great assistant in Carlos Queiroz. He will come into the reckoning," he said. "But we have owners and a chief executive who will choose the new man in three years' time."