The Art Fund, the independent art charity, said its grant towards the purchase of Titian's Diana and Actaeon, which hangs in the National Galleries of Scotland, was its biggest ever for a single painting and the campaign's first major donation.

The fund hopes its lead will spur private donors, and crucially the British and Scottish governments, to help buy the work from the Duke of Sutherland.

"Today we have put our money on the table," said the fund's director, David Barrie. "We hope that it will encourage other people to pitch in.

"We are particularly concerned about the response of the two governments, against the background of this dire economic woe."

The Bridgewater collection of paintings by Titian, Rembrandt and others has been owned by the Sutherland family for two centuries. But the current duke is seeking to sell two of the finest works, Diana and Actaeon and Diana and Callisto to "rebalance" his assets.

The National Galleries of Scotland and the National Gallery in London have negotiated to buy one painting jointly for £50 million by the end of this year, and the second for a similar sum in three years' time, if they can raise the cash. This would ensure the rest of the collection remains on loan in Scotland for another 21 years.

The director-general of the NGS, John Leighton, said he was "very pleased" with the Art Fund's gift.

Talks over government funding – seen as increasingly vital as falling markets hit the pockets of foundations and private donors – were ongoing, he said.

Yesterday top artists from David Hockney and Damien Hirst to Douglas Gordon, the Turner Prize-winning Scot, joined the campaign.