Duke says he is the loser in £100m Titians deal

The duke, in an exclusive interview with Scotland on Sunday, claimed he could have received much more for the works – Diana And Actaeon and Diana And Callisto – had he sold them on the open market.

Frances Ronald Egerton, the seventh duke, also vented his frustration at the time taken to complete the deal, which he said he expects to finalise shortly once the legal paperwork has been completed. The sale comes after a high-profile campaign by Britain's art establishment to raise the 50m for Diana And Actaeon. Diana And Callisto is expected to be sold to the nation for a similar sum in 2013.

The National Gallery in London, which will share the paintings with the National Galleries of Scotland on a five-year rotating basis, has donated 12.5m, the National Heritage Memorial Fund has given 10m and the Art Fund has added 1m. The Scottish Government has said it has made a "significant funding pledge" to help save the masterpiece from being lost to a private collector, but has so far refused to confirm reports that it has put up 17.5m.

By selling to a public collection, the duke, who has an estimated fortune of 230m, will avoid inheritance and capital gains tax on Diana And Actaeon, which is regarded as one of the finest Renaissance works in private hands.

The picture is to be sold to the nation under a private treaty sale, a scheme that offers tax breaks to encourage private owners of "pre-eminent" works to sell them to public galleries. Experts have estimated that the painting could have fetched 150m if it was auctioned privately, a scenario that would have left Sutherland with a tax bill of 60m.

Sutherland said: "From the valuations and things being bandied about, I don't know if I'm doing that well (out of the sale]. I think the nation is probably doing pretty well.

"I'm sure that the figure is a good deal lower than it would have been. It is 50m for the first of them and there was a price in August of 300m for the two of them (on the open market]."

The Duke said he hoped the deal would be signed in a "week or two", but added: "I have been frustrated by the delays. I think we first started talking about this two years ago. It is unfortunate that it has dragged along to this unhelpful time economically. I honestly don't know why it has dragged along."

Asked what he will do with the money, the duke said: "There is not any particular project. It is really balancing up the assets."

Labour politicians have questioned why one of the country's richest men should be receiving so much public money for paintings he inherited. But the Duke responded: "I can't see where class warfare comes to it. They are pictures. They are either worth saving so they remain in the National Gallery in Edinburgh or they are not. That's the only question."

But his suggestion that the paintings were a bargain drew an angry response from art experts and politicians. Professor Barry Fantoni, the artist and art critic, said: "What's an extra million to someone like him? It is obscene. He didn't buy it. It has been in his family a long time and it is going to earn him a lot of money."

Ian Davidson, the Labour MP for Glasgow South West, said: "At this time of economic difficulty, I think it is an obscene amount of money. During a year when we are trying to attract the descendents of Scots back to this country, they will come home to discover that the Duke of Sutherland is getting nearly 20m from the SNP Government. They will wonder what kind of Scotland they are coming back to."

The paintings have been in Sutherland's family for 220 years and have been on loan to the National Galleries of Scotland since 1945. Painted in 1559, they have never been separated.