Sir Alex Ferguson insists Manchester United's Premier League title defence could yet unravel.

United occupy a seemingly unassailable position at the summit, with nearest rivals Chelsea trailing by five points with four matches remaining.

A 1-1 draw with Wigan saw the Blues lose further ground on Monday but Ferguson, speaking at the presentation of a Nationwide Lifetime Achievement Award to Denis Law, remains cautious.

"Wigan holding Chelsea puts us in a good position but in my experience in life you don't take things for granted," he said.

"We've got some difficult games ahead - Blackburn, West Ham, Chelsea and Wigan. We have three away games and one at home so we have to do a job.

"Because we have difficult games left, there is still a lot to play for and it could still go down to the wire.

"We could lose to Chelsea and we could lose all our remaining games. It's a very difficult league these days."

United have been ruthless this season, scoring 19 more times than Chelsea for a superior goal difference that is virtually worth a point.

Winger Cristiano Ronaldo has led the charge with a stunning 38-goal haul and Ferguson claims United's ambition deserves the return.

"One of the encouraging things is the goal difference is helping us," he said.

"That's deserved because we've attacked all season and have put an attacking emphasis on our game.

"Cristiano Ronaldo, Wayne Rooney and Carlos Tevez have scored over 70 goals between them. That's a great return from three players."

United put one hand on the title with Sunday's 2-1 victory over Arsenal and Ferguson admitted the competition has been ferocious this season.

"It's been a really competitive league," he said.

"Arsenal and ourselves spent a long time competing for that top spot and Chelsea have crept us behind us.

"It's very easy to dismiss them now but when we beat Arsenal we knew we were right back in the ball park of going for it."

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