A secretive Middle East consortium has bid £1.5bn for Manchester United only to be firmly rejected by the club's owners, the Glazer family, it emerged last night.

According to information passed to the Guardian this was one of several covert offers which have been made for the club, all valuing United at more than £1bn and all of them rebuffed.

Despite the antagonism facing them from United fans, the rejection of the bids appears to signal the Glazer family's seriousness about not selling the club – at least for the medium term. The Glazers are now talking about staying at Old Trafford for another 10 years.

The news will come as a blow to the Red Knights, the group of businessmen who have been preparing a bid for United and have carried out their activities more publicly. The biggest concern will be that the apparent valuation that others put on the club far exceeds their own, which has been put at around £800m.

The offer from the Middle East that valued the club at £1.5bn was made towards the end of last year. It was flatly turned down and the bidders were discouraged from returning with a second offer, despite being willing to pay significantly more than United's £1.19bn valuation in the Forbes football rich-list.

The revelation represents depressing news for those fans who have been campaigning for the Glazers' removal and had hoped that the hostility shown towards the Americans would help to persuade them to sever their ties with the club. More than 150,000 people have joined the Manchester United Supporters' Trust, the group co-ordinating the anti-Glazer movement, and the protests have become increasingly voluble since the release of a bond prospectus in January that laid bare the Glazers' business model.

The Glazer family are said to be unmoved by the animosity and thick-skinned enough not to allow it to affect their planning. They are described as enjoying the prestige of being associated with a winning team. That paints a bleak picture for the former United director Jim O'Neill, now the chief economist at Goldman Sachs, who had been hoping to move into power at Old Trafford via the Red Knights, the group of "high net value individuals" that also includes the former Football League chairman Keith Harris.

An offer from the Red Knights is anticipated in the coming weeks but, even if it is substantially higher than the £800m initially discussed, the Glazers will reject it out of hand and offer no indication of a price that might tempt them to consider a sale. This could be seen as a negotiating tactic, but the Glazers' message is "thanks but no thanks".

O'Neill and his associates have already inquired about the Glazers' position via Rothschild, the investment bankers who helped the Americans with their takeover in 2005, and were told that the club was not for sale. That position is unlikely to change. "It's like having the most beautiful house," according to one top-level source. "They could sell that house but why would they want to sell it if it were the most beautiful house around?"

On the contrary, the Glazers are already thinking far enough ahead to be talking about refinancing their debts in 2017. They accept they could have been more open with the supporters and are aware of the misgivings about the £700m worth of debts they have brought to the club. They also hope to be at Old Trafford more next season. Avi Glazer has been a regular visitor but his brother Bryan has found it harder because his children are younger.

But they reject criticisms about the average 48% rise in season-ticket prices, pointing out that Old Trafford sells out for every game and is cheaper than Arsenal, Chelsea and Tottenham Hotspur. Many supporters are planning to boycott the club next season, effectively going on strike until the Glazers have moved on, but the Florida-based owners are said to be relaxed about the possible financial implications, believing it will make little difference to their annual results.

They have also reiterated to Ferguson that he can spend significant sums in the summer transfer market if he wishes. High-ranking sources at Old Trafford have confirmed that Ferguson is satisfied with the financial position and is genuine when he says he has a strong relationship with the Glazers. However, he has resisted the temptation to pay exorbitant prices and has adopted a policy of bringing in young, up-and-coming players.

The Glazers, nonetheless, are satisfied with the financial backing they have given Ferguson during the last three title-winning campaigns, which also brought two Champions League finals, and believe they would not have come under such scrutiny had it not been for the huge sums of money being spent at Manchester City under the Abu Dhabi United Group.

They believe the criticisms of their regime have been exaggerated and have taken encouragement from the fact their bond issue, raising £504m, was twice over-subscribed. They are also unimpressed with the manner in which the Red Knights have waged an aggressive media campaign aimed at undermining their ownership and attracting support from the club's phenomenal fan base.