MUSEUMS and galleries will watch helplessly this week as a pair of historic paintings of London by Canaletto, the 18th-century master, leave Britain.

View of the Grand Walk, Vauxhall Gardens and Interior of the Rotunda, Ranelagh offer a record of London history that is long gone. The fashionable Vauxhall Gardens that Canaletto depicted in 1751 are now dominated by tower blocks.

But despite the importance of the pictures, they will be exported overseas because cash-strapped public collections have been unable to save them.

These losses are the latest in a long line of treasures that have left British shores. Recent figures show that, by value, only 12 per cent of objects stopped at export were saved in 2004 because museums and galleries are crying out for funds.

The trustees of the National Gallery have issued a desperate plea for help to the Culture Select Committee. In a submission, seen by The Times yesterday, they say that the works of art, including those on long-term loan to British galleries, are at risk because their owners face such a temptation to sell.

They are calling for the Government to establish a national acquisitions fund, for more lottery money to be used to save the nation’s treasures and for tax incentives to encourage gifts of works of art during people’s lifetimes.

The trustees were particularly frustrated that they were losing a 1515 masterpiece by Titian, the greatest painter of Renaissance Venice, because its owner, the 3rd Earl of Halifax, had decided to sell it. Portrait of a Man, loaned to the gallery since 1992, has been in Britain since the early 18th century and “has always been regarded as one of its greatest works of art”, the trustees said. It is reportedly being marketed for about £69 million. “Removed from the walls of the National Gallery, (it) will almost certainly be sold abroad,” they said.

Even the £6 million needed to save the pair of Canalettos was too much — even though experts had advised David Lammy, the Culture Minister, to place a temporary stop on their export. The deadline for matching the purchase price expires tonight, and no public institution is expected to come forward — despite the popularity of the Canaletto in Venice show at the Queen’s Gallery in Buckingham Palace.

The Fitzwilliam Museum, in Cambridge, is also facing the loss of an important painting by François Boucher, the 18th-century French master, The Times has learnt. It had been on loan for some 11 years, but its owners now want it back. Duncan Robinson, its director, said that he suspected that they would sell it and that he feared such losses would be increasingly common: “We’re very vulnerable at the moment nationally. I see very little hope unless the tax laws can be altered.”

In 2004 about 25 objects, worth a total of £46.4 million, were initially refused a licence by the Government’s advisory body, the Reviewing Committee on the Export of Works of Art. Of those, only nine objects, worth £5.6 million, were saved.

The committee looks at whether an object is so closely connected with national history and life that its departure would be a misfortune and whether it is of importance aesthetically or for study. It can only place a temporary stop on an export and wait for an institution to buy the piece. Other nations have tougher restrictions. Under Italian law, nothing more than 50 years old can be exported without a licence.

Sir Hugh Leggatt, the former Museums and Galleries Commissioner, wrote last week to Gordon Brown, the Chancellor, to suggest that such treasures should be subject to “new, far-reaching fiscal arrangements”. He said that a ban on exports would be unacceptable but that “a substantial tax should be levied on any such object which leaves this country”.

LOST TO THE NATION: WORKS OF ART SOLD OVERSEAS

Pair of Georgian mahogany side tables, made for Hagley Hall, Worcestershire, one of the finest interiors of its day (£1.2 million), sold in 2001 to a New York dealer