Image caption John Sweeney followed the debt trail of the Glazer family

Manchester United's owners are £1.1bn in debt - £400m more than previously known - after borrowing extensively against their shopping mall business.

BBC Panorama has found evidence that the Glazer family's debt levels may threaten their hold on the club.

A spokesman for the American family has said it holds more than £2bn in assets.

But the extent of the debt owed by the Glazers is likely to fuel a continuing revolt by some supporters, who oppose their ownership of the club.

Green and gold

Details of the financial arrangements of the owners also come at a time when the sport's governing bodies are facing questions about Premier League debts that have reached a combined total of £3.4bn and the growing popularity of leveraged buyouts in English clubs.

These are people who tell us not to worry about Manchester United debt because they are great businessmen. In their core business in the US they got it absolutely wrong Andy Green, City Analyst and Man U supporter

Mortgage documents seen by the BBC show that the Glazers have borrowed £388m ($570m) against shopping malls and £66m ($95m) against their American National Football League team, the Tampa Bay Buccaneers.

In addition to their mortgages in the US, a portion of the Glazer family's £700m Manchester United debt will soon see them charged interest at a rate of 16.25%.

Fans fear that, despite the club's record of success on the pitch, the Glazers' leveraged buy-out of United has saddled the club with debt and that may mean that there is no spare money in the future to buy a new generation of star players.

Disappointed fans have launched the "green and gold" campaign that resurrects the original team colours in protest over the Glazers' ownership.

Their numbers have reached 158,000 and former United star David Beckham has signalled his support.

They point to the £80m sale of star striker Cristiano Ronaldo last year and note that he has not been replaced by a player of similar quality. Yet ticket prices have gone up by more than a third.

The club's management denies any lack of commitment to buying new talent and says that cash is available for Sir Alex Ferguson to buy players.

Negative equity

City analyst Andy Green, 37, is the disgruntled Manchester United supporter who first uncovered the extent of the Glazers' debts.

Mr Green said: "They borrowed more money at inflated valuations right at the top of the cycle.

Image caption The Glazers have defended their ownership of the team Man United's detective super-fan

"These are people who tell us not to worry about Manchester United debt because they are great businessmen. In their core business in the US they got it absolutely wrong."

The debt levels at the club are also drawing the attention of other prominent football figures.

Dave Whelan, Chairman of Wigan Athletic, told Panorama: "I don't think anybody can be satisfied with how Manchester United are being run... they have got somewhere in the order of three-quarters of a billion pounds worth of debt. That has got to be eliminated and eliminated quickly."

The Glazer family's main assets are the shopping centre business in America, First Allied Corporation, along with Manchester United and the Tampa Bay Buccaneers.

First Allied is a private business and its accounts are not publicly available. But Mr Green discovered that the Glazers' shopping mall mortgages had been bundled with other loans as Commercial Mortgage Backed Securities.

Those bundles are publicly traded and therefore require the Glazers to provide detailed information on all the mortgages, which are then publicly available in the US.

Mr Green found mortgages - confirmed by the BBC - on 63 of 64 First Allied shopping centres, totalling £388m ($570m).

Most of those were taken out with Lehman Brothers before the US investment banking giant went bankrupt, triggering the global banking crisis in 2008.

'Watch list'

While Lehmans collapsed, the Glazers' mortgage debt lived on and many of those shopping centres are not generating enough income to keep up with interest payments.

With falling commercial property values, many are also now in negative equity.

Banks have put 28 of the shopping centres on a watch list, meaning they are worried about the loans.

Four shopping centres - one each in Ohio, New Mexico, Texas and Georgia - have already gone bankrupt.

When they bought Manchester United in 2005, the Glazer family borrowed £500m and paid the remaining £272 million in cash.

Mr Green found that the Glazers had remortgaged 25 of their shopping centres in the six months before the takeover.

He believes the family borrowed against their US properties to pay for United: "At the time when they had to present a huge amount of cash over here in the UK they borrowed a huge amount of extra money in the US and publicly they didn't buy anything else that year."

A spokesman for the family did not respond to questions about the mortgages taken out by First Allied.

But with properties now worth about £380m ($550m) but mortgages valued at £395m ($570m), the shopping mall company now appears to be worth next to nothing.

'Commercial expertise'

That financial picture has analyst Mr Green questioning how the Glazers will service their £1.1bn debt.

Image caption Fans are wearing green and gold in protest over the Glazers' ownership

United chief executive, David Gill, has said: "We're very confident the business model we have in place will ensure the club can continue to compete at the top of football for many years to come.

"The owners have been true to their word since they took over the club in 2005. They've brought commercial expertise and commercial benefit to us in a numbers of areas, and we've seen our revenues grow significantly."

The Glazers' most troublesome debts are held by Red Football, the parent company that owns Manchester United.

They are payment in kind loans, or PIKs, worth £200m and the interest owing on them will soon rise to 16.25%.

Mr Gill told the BBC in January: "We don't worry about the PIK repayment. That's nothing to do with the club."

A spokesman for Manchester United told the BBC last week that the club stands behind Mr Gill's assertion that the debts will be repaid without involving the club.

But sources close to the Glazers have confirmed that Red Football may use cash from Manchester United to pay off the PIKs in the future. The Glazers are said to be "comfortable" with the PIKs.

The situation at Manchester United reflects the wider issue within the Premier League, where clubs like Liverpool and West Ham are struggling with huge debts and FA Cup finalists Portsmouth barely staved off bankruptcy.

Both the Premier League and the FA declined requests for interviews on the subject of debt in football.

Panorama: Man United - Into the Red, BBC One, Tuesday, 8 June at 2235BST and then available on in the UK on the BBC iPlayer.