Watkins is thought to have been in the house when the attack occurred, with paint also being sprayed on an Audi and over parts of his house.

Greater Manchester Police have been informed of the incident, which has staggered United officials.

“Manchester United condemns the acts of vandalism committed against one of its directors, Maurice Watkins, yesterday,” said a club spokesman.

“The behaviour of a few individuals does nothing to enhance the reputation of the club they claim to support.”

Watkins is thought to have been targeted because a million United shares he sold in April ended up in the hands of American business tycoon Malcolm Glazer, the man widely believed to be behind the `preliminary approach' to buy the club, which was confirmed to the Stock Exchange yesterday morning.

United supporters groups are opposed to any takeover in the belief the billionaire owner of the Tampa Bay Buccaneers NFL franchise would substantially raise ticket prices in an effort to pay off the hefty loan they think he needs to acquire the club, whilst also maintaining a healthy profit margin,

Demonstrations against Glazer were held prior to Sunday’s Barclays Premiership encounter with Middlesbrough and some fans vowed to ‘fight dirty’ to stave off Glazer’s advances.

Watkins was unavailable for comment today but the attack will no doubt have come as a major shock to one of the most respected figures in the game.

Apart from retaining a 1.9% share holding, worth an estimated £13m (€18.8m), Watkins has also been heavily involved in representing United players in a series of disciplinary cases and was the man who publicly announced United’s intention to fight Rio Ferdinand’s eight month ban for a missed drugs test.

He is also a member of the UEFA external legal experts panel and the FIFA dispute resolution chamber and played a leading role in drafting the new transfer regulations that came into force four years ago.

It appears he has now become the latest victim of the militant section of the United support prepared to take direct action to prevent a takeover.

Their previous tactics included staging a racecourse protest at Hereford earlier this year in response to the intervention of major shareholders John Magnier and JP McManus, who will be pivotal to Glazer’s chances of taking control.

The Irish racing duo knew nothing of yesterday’s announcement but with their own stake standing at 29.3% agreement is essential for Glazer to stand any chance of succeeding in his aim.

United are currently waiting for clarification of the initial proposals, which saw the club placed into an ‘Offer Period’ for the second time this year.

At some point, Glazer will either be forced to announce he has made a bid or inform United chief executive he is not, which would preclude him from making another attempt to seize control for a further six months.