Fears that a diminutive 495-year-old masterpiece by Raphael could be sent abroad increased yesterday after the Government made a surprise announcement that it needed more time to decide its value.
A final decision on the proposed £34.9m sale of the Madonna of the Pinks by the Duke of Northumberland to the Getty Museum in California was due next week after eight months of wrangling by London's National Gallery to keep the work in Britain.
The saga, which has brought one of the country's richest landowners into conflict with the art establishment, seemed close to a conclusion last month when the Heritage Lottery Fund said it would give £11.5m towards buying the painting from the duke's priceless art collection, which also includes Canalettos and Titians.
But the Department of Culture, Media and Sport (DCMS) said yesterday it was still calculating if the National's £21m offer was sufficient compensation to the duke - possibly leading to a new export ban once the current deferral expires next week. A spokeswoman said the complexity of the case and numerous outstanding issues meant a decision may not be possible by the deadline.
The delay will raise concern that tax experts consulted on behalf of the Arts minister, Estelle Morris, have not agreed with the painting's valuation by the National and its supporters.
Under the rules governing the export of art, a work can only be retained if a "matching offer" is made by a British public institution equal to any made by a foreign buyer.
The National claims its valuation of £21m - some £8m less than the Duke of Northumberland claims he would receive from the Getty after tax - is a matching offer because of an outstanding dispute over capital gains tax.
The problem arises from a valuation of the Raphael of just £10,000 in 1988, when the painting was thought to be a copy. Once the work, painted in 1507 for an impoverished nun, was recognised as genuine its value rocketed, making the duke liable for £8m more tax, says the National.
Officials admitted yesterday that the decision lay in the balance. One DCMS source said: "It could require a further deferral or we may have to sign the export licence."
The duke said yesterday he needed the full sum offered by the Getty as he was proposing to put the sale proceeds towards the upkeep of the £300m estate under his ownership, which includes the ancestral homes of Alnwick Castle and Surrey's Syon House.
But a spokeswoman for the gallery said: "We remain confident that £21m is the appropriate compensating offer."