Few heirs come more reluctant than the 27th Earl of Dumfries. Johnny, as he likes to be known, is a direct descendant of Robert the Bruce and has never been adverse to selling the occasional heirloom. Now he has upset the guardians of Scotland’s heritage with the most dramatic rejection of his aristocratic roots so far.

Today, the 18th century Palladian mansion he inherited as the Marquess of Bute will go sale, along with its entire contents, including the country’s finest complete ensemble of Chippendale furniture, which will later be offered at auction.

Not only will the sale end 250 unbroken years of family history, it will also dissolve a rare and remarkable collection of art, architecture and furnishings, which has survived almost unaltered since their Rococco heyday in the 1750s.

The invitation for a private buyer also brings to an end efforts by public heritage bodies over the past three years to save the unparalleled Scottish collection for the nation.

National Trust Scotland was invited to buy the estate in 2004 but, to the “extreme disappointment” of the chief executive at the time, failed to secure enough funds.

Mr Dumfries said yesterday that he had decided to sell the national treasure as part of a plan to restructure his finances.

The former Formula One racing driver, who has already sold off his father’s yacht, King Duck, and more than £250,000 of items from his home, Mount Stuart House, in the Isle of Bute, said that he wanted to concentrate on the island estate rather than the category A-listed mansion in East Ayrshire, which was last occupied by his late grandmother.

He said that Dumfries House, with its new price of at least £6.8 million, “is an ideal asset to dispose of, as it is a relatively small, well-managed and profitable estate”.

If sold together with its unique collections of George II four-poster beds, Gobelin tapestries and, most importantly, the wealth of mahogany chairs, tables and bookcases that came straight from Chippendale’s workshop, the estate will fetch at least £20 million.

But conservation experts point out that buyers willing to keep the whole collection intact are few and far between.

With Savills instructed to market the 1,940-acre property, and Christie’s processing brochures detailing the individual lots, it is feared that the collection’s greatest asset as a complete time capsule of the Age of Enlightenment will be lost.

“It would be a tragedy if, after 250 years, a collection of such importance were to be dispersed,” said Michael Hitchon, of the local heritage body, the Kyle and Carrick Civic Trust. “[Dumfries House] is a rare, rare survivor. But I imagine there will be few private owners who have the finances to buy the whole shooting match.”

He said that selling to a private buyer would be a missed opportunity in an area of such underused tourist potential.

“It was very unfortunate that the owner did not reach an agreement with the National Trust, because a public appeal would have had real support. There is a strong argument for Historic Scotland to acquire the whole estate, lock, stock and barrel for the nation. It would be a wonderful asset.”

Dumfries House has never been open to the public.

Thirteen miles from Ayr, it was designed and built by Robert and John Adam, sons of the great Scottish architect William Adam.

Much of its interior was commissioned specifically for the house, including a rosewood Chippendale bookcase, which is expected to break all British furniture records by fetching up to £4 million during the two-day auction in July.

The sale, which is sure to prompt an international feeding frenzy among dealers, has also raised questions over who should accept responsibility to preserve such privately owned national treasures.

Mr Dumfries’s stepmother, Jennifer, Marchioness of Bute, called yesterday on public bodies to step in to save the estate from asset-stripping at auction.

“This house should not be broken up and sold off piecemeal,” she told The Times. “I know that the National Trust doesn’t have the money, but it could have been the inspirational saviour who got all the others — Historic Scotland, the local associations — on board.

“It need the public authorities to show some energy, show some leadership. Otherwise, it’s going to be very sad.”

Lady Bute dismissed the argument that the public should not pay for an estate whose doors, no matter how beautifully designed, had always remained locked. “That is totally irrelevant,” she said. “Many of the other big houses saved for the nation have remained private.”

A spokesman for National Trust Scotland said that the estate was simply out of its league: “At present the Trust is not in a position to commit the level of expenditure required to purchase Dumfries House without impacting upon the duty of care to the current properties and collections in its portfolio. The National Trust for Scotland would be pleased to be involved in Dumfries House by offering advice and guidance to the current and prospective owners to ensure this property remains a significant asset to Scotland’s heritage.”

The auction will take place at Christies on July 12 and 13.

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