www.scotsman.com The centrepiece collection of the National Galleries of Scotland could be broken up unless it can raise £100 million to buy two Old Master paintings.

The 7th Duke of Sutherland, owner of 28 Renaissance masterworks which have hung in Edinburgh for 60 years, has decided to sell major works to "rebalance" his family's assets.

But he has offered the National Galleries a deal: if it buys two Titia ns for £100 million, the collection will stay in Scotland for the next 21 years.

John Leighton, the director-general of the galleries, described the situation as both a crisis and an opportunity. He said it could secure the future of the famed 200-year-old Bridgewater collection of works by Titian, Raphael and Rembrandt for a new generation - or see significant works put up for grabs to collectors across the world.

Losing the Bridgewater paintings would be "like the Uffizi without its Botticellis, the Louvre without its Mona Lisa", Mr Leighton said. "Whereas our Michelin rating would be 'must see', we would fall down the ratings to 'not worth a detour'.

The Bridgewater collection of 27 Old Master paintings and one Rubens drawing was put together by the Earl of Bridgewater in the late 17th century. Passed down to the Sutherland family two centuries ago, it was brought to Edinburgh for safekeeping during the Second World War and has stayed here ever since.

It was credited with turning the National Galleries of Scotland (NGS) from a small-scale collection into a world player. But staff have always feared for a collection that depended on the owner's goodwill - and is now thought to be worth more than £1 billion.

The deal the NGS has been offered involves buying Titian's Diana and Actaeon for £50 million by 31 December. The galleries would then have to buy a second Titian - Diana and Callisto - for a similar sum four years later. Both purchases would be made jointly with the National Gallery in London. The deal, cast as a bargain-basement price, would secure the loan of all the paintings for the Scottish galleries for the next 21 years, although the two jointly owned paintings would alternate between Edinburgh and London on a five-year cycle.

If the deal falls through, however, the Sutherland family is set to take the Titians or other major works to the open market, saying they will pursue "all options as an owner".

The NGS stressed it would be getting the Titians at a bargain price, with the respected Art Newspaper saying the pair could fetch £300 million on the open market.

No immediate reason was apparent for the 68-year-old duke's decision to sell, but he has two sons, who are both married with children of their own, and there was speculation that inheritance tax planning could be a motive.

The duke's non-art wealth, including estates in the Scottish Borders and East Anglia, has been estimated at £30 million. By contrast, the Bridgewater paintings are estimated by the Art Newspaper to be worth more than £1 billion - even without the two Titians, considered the finest works.

The duke's spokesman said the collection had "grown in value to the point where it is prudent to review the holding in terms of the balance of the family's overall assets". He added that it was "sensible" to sell part of it, with the Titians offered to the nation at below market price.

He went on: "There is the prospect he would sell paintings on the open market. It might be these, it might be others. If the deal doesn't come off, he retains all his options as an owner."

The Bridgewater collection also includes two Raphaels, a Rembrandt self-portrait, four Titians, a Rubens, a Tintoretto, and Poussin's famous Seven Sacraments, and they have a room of their own in the National Gallery.

In 1984, the NGS bought four paintings from the collection.

In 2003, when the 6th Duke of Sutherland died, the galleries acquired Titian's famous Venus Anadyomene for £11 million, which was sold partly to offset inheritance tax.

For decades, gallery bosses have assiduously cultivated relations with the Sutherland family. But never before has a major sale been threatened.

The Bridgewater collection was considered of such importance to Britain that incoming prime ministers were traditionally briefed on its contents.

First shown in public in London in 1806, it transformed the standing of the National Galleries when it arrived in Edinburgh in 1945.

Mr Leighton said: "Say we fail, then they will be sold. I can't speak for the duke - I can't say which works will be sold - but the decision has been made to sell a significant quantity."

He said that, while £50 million was a huge sum, the duke was offering "extremely generous terms" on a deal for "children and grandchildren".

It would be hard, but not impossible, to secure the "once in a century" deal, he said. But if the 31 December deadline was not met, "we are away from the table and out of the room".

Mr Leighton insisted the two Titians were arguably among the greatest paintings in the world and that the duke was offering a "substantial discount" in a private treaty sale that would carry tax relief.

Nicholas Penny, the director of the National Gallery in London, said there was no greater private collection of art. He went on: "The price for the Titians is extraordinarily advantageous for us. We've really got a lot of time. It doesn't mean all the money has to be in their pocket by then."

Martin Bailey, of the Art Newspaper, said: "It's the Renaissance collection that's at the heart of the National Galleries of Scotland collection, and without the Sutherland, the heart is torn out. It's difficult to imagine it happening, but it could.

"It puts the National Gallery of Scotland in a very difficult position.But loans cannot go on for ever, and public galleries have to react accordingly."

Any deal of this scale would typically bring together UK and Scottish Government funding, lottery funding, tax write-offs, and gallery and private funders.

A Scottish Government spokeswoman said: "Scottish ministers are determined to do everything they can to ensure this world-class collection remains on view for the enjoyment of the public in this country."

The NGS has two other multimillion pound projects on its hands, with the D'Offay Collection of contemporary art and the overhaul of the Scottish National Portrait Gallery.