Conservationists fear that, with the Government unwilling to come to the rescue and groups such as the National Trust and English Heritage already financially stretched, many may be closed to the public, architecturally ruined, their estates and contents sold off.
Oliver Pearcey, a conservation expert with English Heritage, said: 'The future of these houses is something that concerns us. It is certainly an issue at the moment. We don't keep statistics but there are large numbers of important, historically and architecturally significant houses coming on to the market at the moment.
'The reasons seem to be a combination of economic recession, Lloyd's underwriting losses and mortgage problems.'
Justin Marking, of the estate agent Savill's country department, said: 'It is certainly true there are a large number of notable Grade I listed properties on the market at the moment.' Four properties he was dealing with included one casualty from Lloyd's underwriting losses and three insolvency cases, he said.
Only a few weeks remain to save one historic house that has fallen victim to Lloyd's losses. Pitchford Hall, near Shrewsbury, described as one of the finest black and white, half-timbered buildings in Britain, is up for sale because its owners, Oliver and Caroline Colthurst, face debts from insurance underwriting losses.
Unless a financial package is put together soon the house will be sold and its contents auctioned separately in September. The owners have offered the house and its 76 acres, which include what is reputedly the world's oldest tree house, to the National Trust as a gift if pounds 1.8m can be found to purchase the contents. But the trust estimates it needs more than pounds 10m to repair and endow the property.
Other conservation groups are equally financially stretched, and the National Heritage Memorial Fund, created by the Government to save houses like Pitchford, has already committed its funds to save another house.
Even if funds were found to save Pitchford Hall, other important houses have come on to the property market. One is the Grade I listed, 18th-century Georgian mansion Heveningham Hall, in Suffolk.
The property was saved in 1970 by the Government, which then sold the house and 469 acres to an Iraqi businessman, Abdul Amir Al-Ghazzi, in 1981. The property is back on the open market with a pounds 4.5m price tag.
Cork Gully, the receivers disposing of the house, consider it so important to the nation that they opened the hall to the public this month. Estate agents acting for Cork Gully say this could be the last opportunity for the public to see the property if it is sold to a private buyer.
Mr Marking said some properties on the market for long periods had been unable to find buyers at their original prices but were attracting considerable interest after prices dropped. Herstmonceux Castle in East Sussex, for which offers higher than pounds 20m were initially asked, had attracted international interest since the price was reduced to more than pounds 5m.
The more modest Hinwick Hall in Bedfordshire had seen its price drop from an opening pounds 2m to its current pounds 975,000. Mr Marking said mortgagees and receivers were very good about maintaining properties and their grounds while they were unsold.
Patrick Ramsey, a partner in Knight Frank and Rutley's country house department, said he thought more imaginative ways of providing financial help to house-owners was necessary. 'Many people who have struggled for years to maintain their outstanding houses are now forced to sell them because of the cost.
'There are jolly few people who will take on a Grade I house. You don't own the house; it owns you.
'So many people seem to think you must be frightfully rich to live in a big house. This is simply not true. Many existing owners live very frugally. Many are far from rich and are really caretakers for the nation, without pay.'
He said better ways of helping house owners had to be found. 'It has to be more economical to help owners preserve their homes.'
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