Manchester United fans have accused the club’s biggest shareholders of running a “dirty tricks campaign” against manager Sir Alex Ferguson.

Horse racing magnates John Magnier and JP McManus have been accused of conducting a “personal vendetta” against the club and manager, according to the Supporters’ Association.

The two men, who own 25% of United, are unhappy about recent transfer dealings and they have forced the club to conduct a review of the all the transfer deals of the past five years.

United pledged on Monday to conduct an internal review, although the Irish duo believe that the terms of the inquiry do not go far enough and believe that the review should be conducted independently.

Magnier and McManus are in dispute with Ferguson over stud fees to the stallion Rock of Gibaraltar.

The two men have sent a letter to United’s board asking 63 questions relating to corporate governance, and are said to be still likely to demand an extraordinary general meeting to discuss the issue.

The Guardian newspaper quoted a source close to Magnier and McManus saying: “The inquiry announced by the club is not enough and, as yet, the major shareholders in United have still not received clarification over the issues they have raised.

“United have done nothing to alleviate the concerns of its main shareholder.”

Press reports say Ferguson is likely to sign a new contract this week, but that it will be the one-year rolling deal that the Irish shareholders have demanded, rather than the longer four-year deal Ferguson had hoped for.

So angry are the Irish duo that it has been been suggested that theywould be prepared to buy the club and sack the baord to ensure they get their own way on this issue.

‘These are very very wealthy men and they are determined to be vindicatedon this issue,’ a source told The Guardian.

‘If they hadto buy the company and sack the board they will. They will have no difficulty in raising £600m. If you sold it today, the horse would fetch £50m at a push.

‘It reflects their fury about this that they are willing to put their £200m stake in United against that.’