The BBC said its investigation found the Glazer family's debts are $570 million greater than previously known. Malcolm Glazer and his sons also own the Tampa Bay Buccaneers.
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All the extra debt comes after the Glazers borrowed extensively against their shopping mall business in the United States. The extent of the debt could fuel a further revolt by a group of United fans who oppose the Glazers' ownership.
The Glazers took over United in 2005 in a leveraged buyout worth $1.4 billion.
The BBC said it saw mortgage documents showing the Glazers have borrowed $570 million against shopping malls owned by their company First Allied Corporation. That's in addition to $1 billion in debts tied to Manchester United and $95 million to the Buccaneers.
The Buccaneers released a statement Monday trying to reassure their fans that the team is not in financial trouble.
"Buccaneers fans should know that the Glazer family is as financially well-positioned as ever before. Companies they own generate revenues in excess of $800 million each year," Jonathan Grella, the team's director of communications, said in the statement.
"Sophisticated real estate experts know that the family's refinancing of their commercial real estate before the global meltdown has proven to be the wise move.
"While First Allied represents only a small portion of their asset portfolio, it continues to generate significant profits, enjoys over 90 percent occupancy, and has long-term non-recourse financing.
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"This franchise remains committed to bringing the resources to build its next championship team."
The Glazers have said they have assets totaling $2.9 billion.
"I don't think anybody can be satisfied with how Manchester United are being run," Dave Whelan, chairman of Premier League club Wigan, told the BBC program "Panorama."
"They have got somewhere in the order of a three-quarters of a billion pounds worth of debt. That has got to be eliminated and eliminated quickly."
The research for the BBC program was carried out by Andy Green, a London financial analyst who is part of a campaign by disgruntled United fans to oust the Glazers.
Thousands of United fans attended matches at Old Trafford last season adorned in green and gold colors instead of the club's usual red. The homage to United's original 19th century colors is a protest against the Glazers.
In March, United chief executive David Gill said the club's debts were easily covered by the income.
"In an ideal world people would like to not have a mortgage on their house, but that doesn't mean they don't enjoy the benefits of living in that house and can't afford that house," he said.
The Glazers have said they will not sell the club.
Forbes magazine recently estimated United is worth $1.8 billion -- making it the most valuable soccer team in its rankings for the sixth straight year.
The Glazer family has said that money will not be an issue in running the Buccaneers.
"Our football people have never been told no because of money,'' Buccaneers co-chairman Joel Glazer told the Tampa Tribune in late March. "Money will never be an issue when it comes to building this team the way we think it should be built."
Bucs officials, however, have said season-ticket sales are down and there is a real possibility of local television blackouts this year. Tampa Bay has sold out every home game since Raymond James Stadium opened in 1998.
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