Mr JP McManus and Mr John Magnier are not engaged in a "Machiavellian" plot against Manchester United manager Mr Alex Ferguson, according to an associate of the two, financier Mr Dermot Desmond.

The pair have raised the stake in United to 28.9 per cent for strictly financial reasons, rather than in a bid to weaken the position of Mr Ferguson, who has had a falling out with Mr Magnier over the ownership of a racehorse, insisted Mr Desmond, a minority shareholder in club.

The Irish magnates "didn't invest to lose" and would not allow emotion to colour their business judgment, said Mr Desmond, speaking to RTÉ radio yesterday.

The row between Mr Magnier and Mr Ferguson over racehorse Rock of Gibraltar, whose stud fees could be worth millions, was "totally different" to their investment in the club, he added.

"People who really don't understand and haven't appraised the situation properly have determined there's some sort of Machiavellian plot here. There isn't," said Mr Desmond.

Mr Desmond's comments will be regarded as significant as he is an associate of Mr Magnier and Mr McManus, neither of whom has commented publicly about the motivation behind their investment in Manchester United.

The pair have raised concerns over the role of agents in transfer deals at United and have sought information from the club on this and other issues.

In an interview with RTÉ radio's Sport Matters, Mr Desmond appeared to share these misgivings, saying agents had fuelled wage inflation in the game. Clubs should refuse to pay agents' fees, he said. A player wishing to retain an agent should foot the bill himself.

The intervention of Mr Desmond, who is the largest shareholder in Celtic FC, came as United fans prepared a "Not for Sale" campaign in an attempt to ward off a takeover of the side.

An American billionaire, Mr Malcolm Glazer, is reported to be considering a takeover, having last week increased his holding to 16.3 per cent. He said in a statement this week that no move was imminent, but did not rule out the possibility that he would make a bid.

"We are saying to all predators out there that the customers don't want you in charge, no matter who you are," Mr Oliver Houston of Shareholders United told a news conference in Manchester.

"There are 70,000 supporters at Old Trafford and 53 million around the world telling you to get lost. That's what we will be doing if you try and get your mitts on our club," Mr Houston said.

Shareholders United and the Independent Manchester United Supporters Association (IMUSA) have joined forces with a number of other supporters' groups to promote the "Not for Sale" campaign.

Fans fear a new owner would try to increase profitability via higher ticket and memorabilia prices. The coalition is urging supporters to buy shares or pledge shares they already hold to Shareholders United, and not to sell up in the face of a takeover bid