The announcement will be made by Chris Smith, Secretary of State for Culture, Media and Sport, as a tribute to the unfailing good taste shown by successive generations of the Harewood family, who have maintained the home.
Harewood has been accorded designated museum status by the Museums and Galleries Commission, recognising it as a centre of excellence and putting it in the premier league of museums in the country.
The house, built west of Leeds in 1759, was designed by John Carr and Robert Adam, is set in gardens created by Capability Brown and has always been a showhouse - attracting the gentry from Harrogate almost as soon as the building was finished. The family continues to live in the house, which is run by a charitable trust.
It contains one of the finest collections of Chippendale furniture, views of the house painted by Turner in 1797, family portraits by Sir Joshua Reynolds, Italian Renaissance paintings and Sevres porcelain.
David Lascelles, son of the present earl, said his father had long resisted the title of museum, feeling it could be a "dead hand" in its ability to attract thousands of visitors every year. But Emma Taylor, museums officer, said Harewood deserved to be the first to receive the accolade of museum as it has stood the test of time.
"Good taste is subjective but Harewood House has, for example, the finest example of Chippendale furniture. Good pieces have been collected consistently by different generations of the family," she said.
Harewood's principal curator, Jane Sellars, was careful to point out that the house is not "pickled in aspic".
Walking through the property's state rooms, which are open to the public, Ms Sellars said: "There was always something happening, always a reason to come back to Harewood."
The designation scheme aims to draw attention to non-national, pre- eminent museums collections in England, recognising the excellence of a collection and the institution that houses it. Harewood House becomes one of 43 designated museums overall.