A Baroque stately home designed by Sir John Vanbrugh said to be one of Britain's genuine architectural masterpieces could be lost to the public for ever unless funding can be found, campaigners say.

In a letter to The Times, more than 20 leading architects and academics, including Lord Rogers of Riverside, Will Alsop and Sir Richard MacCormac, call for action taken to save Seaton Delaval Hall in Northumberland. Described by the National Trust as a forgotten gem, the house is likely to be sold and used as a hotel or private accommodation unless £3 million can be raised. The National Trust has 100 days to buy it and prevent the estate being broken up and sold privately.

Seaton Delaval Hall and its gardens will cost £13 million. Of this, £7 million has been pledged by the National Trust itself, which has applied to grant bodies for a further £3 million.

Vanbrugh (1664-1726), who was a dramatist, adventurer and society figure as well as an architect, is best known for designing Blenheim Palace - seen as the culmination of English Baroque - and Castle Howard.

The architects' letter appeals for public support to raise the final £3 million needed to buy a “landmark work” and calls for donations to protect the “unique spirit” of Seaton Delaval.

Plans for the hall, designed for the diplomat Admiral George Delaval, were drawn up in 1719. It was completed in 1728, and was Vanbrugh's final building. The scale is smaller than that of Castle Howard or Blenheim but the hall still comprised a centre block between two arcaded and pedimented wings. Vanbrugh's sombre design befits the bleak Northumbrian landscape.

Seaton Delaval Hall has been passed down for 300 years through the Delaval family, which has owned the estate since the Norman Conquest. After the death last year of the 22nd Baron Hastings his son approached the National Trust in the hope that it could buy and protect the hall for the public. The 22nd Baron had lived in the west wing, though most of the interiors remain unrestored after fire gutted the hall in the 19th century.

A National Trust spokesman said: “For the next 100 days, we as a nation have the opportunity to buy this wonderful hall and protect it for the benefit of generations to come. Without that final £3 million, the hall will be sold privately and will almost certainly be lost to the public, possibly for ever.”

Liz Fisher, area manager for the National Trust, said: “When fire threatened to destroy Seaton Delaval in 1822, 200 villagers turned out to fight the flames - and it is that public support that the hall needs again now.”

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