Shares in Manchester United rose 5.34% on Monday after the club confirmed it had received a takeover approach.
Speculation about a possible United bid has gone on for months
The club issued a statement after reports in the weekend press revived speculation of a bid from US sports tycoon Malcolm Glazer.
Mr Glazer already holds a 19.17% stake in Manchester United.
However, the club has not yet disclosed the identity of the potential bidder and added that the approach contained a "number of significant conditions".
"The company is seeking clarification of this proposal and at this stage it is unclear whether any offer will be made," the club said.
Shares in Manchester United rose on news of the bid talks, and ended the day up 13.25 pence, at 271p.
Already, the news of the bid has aroused fear in fans that if the company is controlled by one person, ticket prices would rise and they would have less of a voice in how the company is run.
The club recently announced a 30% fall in annual profits to £27m ($48.3m), attributed to a weaker performance on the pitch and the higher cost of player transfers.
If Mr Glazer is behind the bid approach he will have to acquire shares owned by Irishmen John Magnier and JP McManus who own a 28.9% controlling stake.
Mr Magnier and Mr McManus have been careful not to take their Man United shareholdings to the 30% threshold at which an offer to buy out the club's other shareholders would have to be made.
They have always insisted that their shares are held as an investment and not as a prelude to a takeover attempt.
Dutch media tycoon John de Mol, the man behind Big Brother, was once another possible contender, but most of his shares are now in the hands of the Irishmen or Mr Glazer.
And back in December 2003, Ralif Safin, one of the founders of giant Russian oil company Lukoil, was tipped to make a move for the Premiership champions, but no bid emerged.
1928: Born in Rochester, NY
1943: Inherits father's watch business
1950-90 Makes fortune via series of ambitious investments
1995: Buys Tampa Bay Buccaneers for $190m
2003: Buccaneers win Super Bowl and are valued at $671m
2003: Fails in bid to buy LA Dodgers. Ups stake in Man Utd
2004:Gradually ups Man Utd stake to 19.7%
Other potential suitors included Scottish mining entrepreneur Harry Dobson, and Dermot Desmond, the Irish millionaire who owns part of Celtic football club.
Mr Glazer, who has invested £140m in buying United shares, has been at the centre of takeover speculation for a while.
If he does launch a successful takeover, he will inherit a debt-free business and one of the world's best known sporting brands with potential for increased marketing opportunities in the US and Asia.
However, according to financial expert and former chairman of the Football League, Keith Harris, Mr Glazer should proceed with caution.
"It is hard to tell if it looks like a very good investment for anybody other than the Irishmen who - if they sell - would look very smart indeed," Mr Harris told BBC Radio 4.
"He would have to buy them out to take control. I don't know if they would be interested, but they are known as very smart investors."
If Mr Glazer raises his stake to more than 30%, he would have to launch a full takeover bid under UK regulations.
Back in March, Mr Glazer was forced to quash weeks of speculation by announcing that he had no current intention of making an offer for the football club.
As fresh speculation mounted that a takeover by Mr Glazer was on the cards, fans gave the prospect a less than enthusiastic response.
On Sunday, the Independent Manchester United Supporters Association said it would take any steps
necessary to prevent the American from owning the club.
"If Mr Glazer wants a fight, we will give him one. We want to make it clear that he is not welcome or wanted as owner
of Manchester United," said Association chairman Jules Spencer.
Mr Glazer turned the Tampa Bay Buccaneers from an under-performing "also-ran" side into a highly profitably Super Bowl-winning franchise but he upset fans by raising ticket prices. He also threatened to move the franchise away from Tampa if the city wouldn't build the team a new stadium.
Supporters fear that the fans' voice has less chance of being heard if control is concentrated in the hands of one person.
They also fear that Mr Glazer has little experience of the UK soccer world.