The attempt to save Dumfries House for the nation looked to have failed last night, as its auctioneers declared that time had run out for a private sale, with a series of international buyers lining up to bid for the historic country house.

The 250-year-old house, near Cumnock in Ayrshire, and its contents, including one of the world's leading collections of Thomas Chippendale furniture, will be sold for around £20m.

The Marquis of Bute, the former racing driver known as Johnny Dumfries, decided to sell the home to restructure his family finances and concentrate on his other properties, primarily Mount Stuart, on the Isle of Bute.

Last week, Historic Scotland said it would not be giving Save Britain's Heritage (SBH) any financial aid in its bid to purchase the house, and yesterday it was announced that SBH failed to table a formal offer for the house by a June 1 deadline imposed by sales agents Savills and auction house Christie's.

Other offers for Dumfries House, described as one of the country's finest private homes, will now be considered. Its contents will be auctioned off next month.

A statement from Christie's said: "John Bute's intentions regarding the sale have been public since 2004, giving relevant heritage bodies both notice and opportunity to consider and propose an offer. It had always been his hope that a private treaty sale solution might be found."

Charles Cator, deputy chairman of Christie's International, said: "Considerable effort has been made over the last three years by John Bute and Christie's to try to secure an agreement and move forward towards a private treaty sale. Sadly this has not proved possible."

Estate agents Savills said there was "significant" overseas interest in the house. Anne Thomas from the agent said: "The sale is proceeding well, with significant international interest received. Closing date for offers for the property will be set in the near future."

Adam Wilkinson, SBH secretary, said his organisation was not giving up its fight. Last week, it made a public plea to wealthy individuals to help the campaign as the sale of the house reached its "end game".

Mr Wilkinson described Dumfries House and its 2000-acre ground as being one of Britain's most important historic homes.

He said his group had raised £7m so far and was working to secure the rest.

Dumfries House was built between 1754 and 1759 by the John, Robert and James Adam, brothers who were friends of the renowned English carpenter Chippendale.

The auction of the contents will take place on July 12 and 13.