Scotland's national art collection is under threat unless the National Galleries of Scotland can raise £100m in the next four years after the Duke of Sutherland decided to sell key paintings in the galleries on the open market.

The duke is the owner of the Bridgewater Collection, the world-famous collection of Old Master paintings which forms the core of the galleries' exhibits.

His collection includes four works by Titian, three by Raphael, perhaps the most important self-portrait by the Dutch master Rembrandt, as well as Poussin's Seven Sacraments and works by Rubens and Tintoretto.

However, last year the 7th Duke of Sutherland decided to sell a significant part of the collection - which has been estimated at being worth £1bn on the open market - after a "prudent review" of his family's assets.

After talks with the galleries, led by director-general John Leighton, the duke has agreed to a deal that could see the paintings, which have hung on the National Gallery's walls since 1945, remaining in public hands.

The NGS, together with the National Gallery in London, can buy Titian's Diana and Actaeon for £50m, which would then be on display on a five-year rotation between the galleries in London and Edinburgh.

If the funds for Diana and Actaeon are raised by December 31 this year, the joint galleries would then have four years to raise money for a second Titian, Diana and Callisto, also priced at £50m. Then, if both these paintings are bought, the rest of the Bridgewater Collection will remain in Edinburgh, on public display, for a further 21 years.

The NGS decided earlier this year that it stood the best chance of securing the works by collaborating with the National Gallery in London.

On the open market, Diana and Actaeon would be sold for far more than £100m, but Mr Leighton said that the duke had offered a "substantial discount", as the duke is "passionate" that the works remain on public view.

If the December 31 deadline is not reached there are no second chances: the duke will proceed to sell an unspecified number of works in the collection. Currently, the loan of the paintings is arranged so that the duke can give six months' notice before removing them.

Mr Leighton admitted that the situation could be described as a "crisis" for the National Galleries.

Last night, a statement from the Scottish Government said: "Ministers are determined to do everything they can to ensure that this world-class collection remains on view for the enjoyment of the public in this country."

Any funds for the purchases - well outside the financial power of the National Galleries - is likely to come from government coffers, the Heritage Lottery Fund and private donations.

Mr Leighton said: "I think the strong tendency on this is to focus on the cost, and £50m is a lot by anyone's standards. But I think it is important to see this in the context of value: the value of the overall collection, and also the value given to the many generations of people who have seen these paintings since we have had them."

Nicholas Penny, director of the National Gallery in London, said: "The loss of these works would wipe out the international importance of the National Gallery of Scotland.

"As it stands, there is no country the size of Scotland that has a greater collection of art. The NGS is great because it has that group of masterpieces. Its loss would be a disaster for Scotland and the United Kingdom."

A spokesman for the duke said: "The Bridgewater Collection has 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. It does now seem sensible to consider the sale of some part of this collection, and the duke and his family would very much hope that it could be acquired by the nation."

Treasures on loan

The Bridgewater Collection, on loan to the National Galleries of Scotland.

Titian: The Virgin and Child with St John the Baptist and an unidentified Saint
Titian: The Three Ages of Man
Titian: Diana and Actaeon
Titian: Diana and Callisto
Veronese: The Madonna and Child with the Infant Baptist
Rubens: Mercury bearing Psyche in his arms to Olympus, and a drawing of Frans Rubens
Tintoretto: Portrait of a Venetian
Rembrandt: Self Portrait, aged 51
Studio of Rembrandt: A Young Woman with Flowers in Her Hair
Studio of Rembrandt: Hannah and Samuel
Follower of Rembrandt: A Study of a Man's Head
Raphael: The Holy Family with a Palm Tree, The Bridgewater Madonna
Raphael and Studio: The Madonna del Passeggio
After Raphael: The Madonna with the Veil
Nicolas Poussin: The Seven Sacraments - Baptism, Confirmation, Marriage, Penance, Ordination, the Holy Eucharist, and Extreme Unction
Gerard Ter Borch: A Singing Practice
Dutch School: An Old Lady Wearing a Ruff
Anthony Van Dyck: Portrait of a Young Man
Hobbema: Landscape with a View of the Bergkerk, Deventer