Tate in £5m race to save Turner's 'Blue Rigi' for the nation

The Tate has launched its first public appeal in two decades to save The Blue Rigi, one of the finest watercolours by the great artist JMW Turner, for the nation.

The gallery is in a race against time to raise the £4.95m that is needed to match the price that was paid by a private bidder at auction last year and keep the painting in Britain.

Nicholas Serota, the Tate's director, described the painting as an extraordinary work of ineffable beauty, which would be of lasting benefit to scholars and visitors if they could secure it for the gallery.

Ian Warrell, a curator and Turner expert, added: "In terms of the Turners that we would ever want, this is right at the very, very top. This has always been on the list."

To promote the fundraising appeal, the watercolour will go on show for the next two months at Tate Britain alongside its sister works, The Red Rigi, which has been borrowed from the National Gallery of Victoria in Melbourne, Australia, and The Dark Rigi, on loan from a private owner.

The three views of the Rigi mountain, from Lake Lucerne in Switzerland, have been never seen together before, even in the artist's lifetime. He had already sold the Blue and Red before painting the Dark.

The Art Fund charity immediately announced it was making a £500,000 donation to the appeal, one of the largest it has made.

A grant application is to be made to the Government's National Heritage Memorial Fund, a fund of last resort for treasures of national importance, and members of the public are being asked to buy a brushstroke of the watercolour online for £5.

Artists including David Hockney, Anish Kapoor, Bridget Riley and Antony Gormley as well as television stars such as Joanna Lumley have already signed up at www.artfund.org/savebluerigi.

The Tate has the biggest collection of Turners in the world, thanks to a bequest from the artist when he died in 1851 aged 76. But it owns none of the 25 finished watercolours Turner produced in Switzerland in the early 1840s. He revolutionised the medium and these works are regarded as the pinnacle of watercolour art.

The Blue Rigi, showing the mountain at dawn, had been in the same family for 60 years until it was auctioned last June for £5.8m, making it the most expensive British watercolour ever sold.

David Lammy, the Culture minister, has placed a temporary export bar on the painting until 20 March to give the Tate the chance to match the asking price. With tax remission, it needs £4.95m. David Barrie, of the Art Fund, said public appeals were rare. The Art Fund has previously done so for the Macclesfield Psalter and works such as the Leonardo da Vinci Cartoon, and the Tate's last appeal was for Constable's Waterloo Bridge in 1987.

But Mr Barrie said: "Turner was one of the most daringly experimental artists of his time, and with this watercolour in particular he really did stretch the limits. This is an opportunity that absolutely cannot be missed."

John Ruskin, the pre-eminent critic of Turner's time, said of the Rigi works they would be recognised within years "as the noblest landscapes ever conceived by human intellect".

The Tate is finding about £2m of the cost of the painting from its own funds and will be dependent on securing a grant from the National Heritage Memorial Fund, which will meet next month to discuss the matter. Collection boxes will be put in the gallery.