At the height of the property boom in 2006, Shrewsbury's shopping centre was sold for £118m with only six of its 135 units lying empty.
Shrewsbury's shopping centre was sold for a greatly reduced price
The value of shopping centres had soared as money poured into commercial property.
Four years and one slump later, 29 units are empty in Shrewsbury's centre which was sold last month at a price believed to be half of the 2006 figure.
All over the UK, negative equity is now widespread in the whole of the commercial property sector - one informed estimate says it could be as much as £50bn.
In the case of shopping centres, it is now common for centres to be worth less than the cash loaned by banks to allow the owners to buy them during the boom.
"£10bn worth of shopping centres are capable of being put into receivership, should the banks so wish," said Mark Williams, of consultancy DTZ, who charts the buying and selling of shopping centres for BCSC (British Council of Shopping Centres).
He added: "That is one in five shopping centres in the UK.
"I'll stress we don't think the banks will put that volume into receivership."
According to property consultant Andy Lamb, negative equity loans are so prevalent there is even a brand new jargon term in the business, the 'Scooby loan'.
Mr Lamb said such loans refer to companies who re-financed their loans when the market rose but found themselves in negative equity when it fell.
"The income is just about servicing the debt....the loan and the loan values are completely underwater and that is called a scooby loan," he said.
"What it means is a scuba diver i.e. he's underwater but he's still breathing, just!
"Because he can meet the interest payments, all other banking criteria are shot to pieces."
William Newsom of property consultancy Savills has been trying to estimate how much negative equity there is in British commercial bank loans.
Basing his research on six monthly data compiled by DeMontfort University Leicester, Mr Newsom estimates a figure of £38bn worth of negative equity on investment properties. But this could rise to £50bn if the drop in land values on development property is considered.
"It is enormous," he said, "but it could be significantly more."
"It has never been seen before to this order of magnitude. It is 25% of the entire lending book in round terms."
Such losses would devastate the balance sheets of the banks who offered the loans, according to Mr Newsom, who says banks are doing all they can to help under-pressure borrowers keep afloat.
The alternative is that if the company goes bust, the losses on the loans will turn up on the bank's accounts.
"There a lot of banks out there who are burying their heads in the sand and not admitting the extent of the problems that they have, for obvious reasons," said Mr Newsom.
The could be up to £50bn worth of negative equity in the sector
But the British Bankers' Association rejects claims bankers are hiding their losses.
"UK banks are subject to one of the most open and robust accounting regimes in the world and the full extent of their liabilities is open for everyone to see when they publish their accounts annually."
The association added: "Banks make commercial decisions about the loans they grant and the repayment terms they impose. They lend based on the ability to repay.
"For large scale commercial property deals terms and conditions are individually negotiated and unique to that project."
Banks have been renegotiating loan terms and extending loan periods hoping that the problems would be resolved in what is known as "extend, amend and pretend" or "delay and pray".
Conor Downey, a specialist banking and commercial property lawyer with city firm Paul Hastings said not all loans could easily be extended.
During the boom a large number of so-called securitised loans were issued for property investors, and he said they were far less flexible.
He warned that a spate of forced sales was likely when these complex loans mature in the next two years.
"It is estimated there are 150 of these transactions across Europe, and about half are British.
"Each one runs to hundreds of millions of pounds in some cases billions," he said.
He said securitisations accounted for between 15 and 25% of all property lending at the peak of the market.
As the number of these forced sales grows this will further depress prices in the commercial property sector making it harder for banks to cover their losses, squeezing the cash they have available to lend to businesses.
"Banks are required by government to maintain certain amounts of cash....to protect them against the risk that a very large amount of their investments go bad.
"As these loans go bad the amount of capital they have available to the banks will decrease unless they can raise it again on the markets to in the long term it will reduce the ability of banks to make loans at the same levels as they have historically," said Mr Downey.
Economist Roger Bootle said banks were in a double bind, "I think it looks pretty bad whatever they do."
He added, "I don't think there is any way the government can dig the banking sector out of the difficulties with regard to commercial property whatever happens.
"There have been some pretty duff loans, the banks are going to register some pretty nasty losses and it's not really a big surprise because it was a massive boom stroke bubble and someone's got to pick up the tab."
File on 4 was broadcast on BBC Radio 4 on Tuesday, 9 February , at 2000 GMT, repeated Sunday, 14 February, at 1700 GMT. You can listen via the BBC
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