The American owners of Manchester United insisted the club was not for sale following reports that a group of City financiers were looking to buy up the Premier League side themselves.

Well-known football deal-maker Keith Harris - a lifelong United fan - is said to have joined forces with the likes of chief economist of Goldman Sachs Jim O’Neill and lawyer Mark Rawlinson to discuss a potential bid.

The group has been dubbed the “Red Knights” and their move comes amid growing unrest at the club’s ownership by the Glazer family and the resulting debt levels.

According to the latest accounts that were released in January, debts at United’s parent company Red Football Joint Venture have increased to £716.5 million and £68.5 million was paid out in debt interest during 2009 alone.

The Glazers also recently launched an operation to refinance £500 million worth of debt associated with the club, prompting fresh concern about the club’s finances.

According to reports, the Red Knights’ hope is to try to starve the Glazers of cash and persuade them to accept a deal that would net the Americans around £1billion.

But a spokesman for the US family said: “Manchester United is not for sale.”

Mr Harris told the BBC the Red Knights’ interest was serious.

He said: “What we don’t know is whether the Glazers can be made to listen.

“What we do know is that there is a serious intent on the part of those people who have not just support in their hearts, but the ability to muster that support in their pockets to get after this.

“The time feels right.”

The Manchester United Supporters’ Trust (Must), which has mounted a vocal campaign to bring about a change of ownership, is understood to back the Red Knights.

Must’s “Green and Gold” campaign has seen supporters don the colours of Newton Heath - the club which was renamed Manchester United in 1902.

There were plenty of green and gold scarves in evidence at Sunday’s Carling Cup final, which Manchester United won 2-1 to clinch manager Sir Alex Ferguson’s 34th trophy during his stint at the club.

The latest revenue “rich list” for the top European football clubs showed Manchester United lying in third place, with £278.5 million during 2008/09 excluding transfer fees.

Spanish giants Real Madrid and Barcelona were top and second with £341.9 million and £311.7 million respectively, the research by Deloitte found.

The next highest-earning English club was Arsenal, with £224 million of revenue.