Rival groups from China and Dubai are gearing up to launch moves for United in the next few months.
And they know the Glazer family — which owns the Old Trafford club — won't even consider any offer below £1bn.
The Chinese consortium have already had an offer of £850million turned down earlier this year-as Sport of the World exclusively revealed at the time.
But they are refusing to be discouraged and will be back with another attempt to own the world's biggest football club, which has a massive commercial market in Asia.
The Arab group — with connections to the Emirates royal family — have kept a low profile but are now ready to make their own move.
A United source told us: "The Chinese people literally have billions behind them and the Dubai group are not far behind.
"It could turn into a bidding war and that will push up the price. That will leave the Glazers facing huge pressure to sell.
"They have an enormous debt which was raised when they bought United and the temptation to cash in and make a profit of over 295m will be hard to resist."
A financial insider added: "The Glazers have publicly stated they are not interested in selling but even their resolve will be tempted by an offer close to 1.5bn.
"It's a phenomenal sum for a football club but it reflects fairly accurately United's worth.
"No other club has such a mass appeal as United.
"Globally, they have an incredible attraction and that's why it's no surprise a Chinese consortium are waiting in the wings. Dubai has already shown interest in Liverpool but United are the ultimate prize — and there will be influential people in that part of the world who would consider it a shrewd investment even at 1.5bn."
The Glazers paid 1.17bn for United after a long and controversial takeover more than two years ago.
And despite the suspicion surrounding the deal, they have consistently backed Sir Alex Ferguson in the transfer market — with United's spending this summer alone hitting the 80m mark.
A spokesman for the family said: "I can state unequivocally that Manchester United are not for sale.
"The Glazer family is 100 per cent committed to long-term investment in the club. Their commitment is underlined by the expenditure during the transfer window. The position has not changed and nor will it."
The Americans, though, have recently tried to restructure 976m worth of loans — much of it to high-interest US hedge funds.
But after talks with several leading banks, including JP Morgan, Royal Bank of Scotland and Deutsche Bank, they were forced to abandon the proposal because of the uncertainty surrounding global credit markets.