For such a costume to have survived from around 1610 complete and in superlative condition is rare enough; for it to have survived with a contemporary portrait is 'doubly rare', if not unique. Often there is no history behind a costume.
Susan Mayor, director of the costume and textiles department of Christie's South Kensington, said: 'It is as if we have a photograph of it . . . Whenever we have early pieces of costume, we are asked how it was worn. With this portrait, we can see exactly how the doublet was worn - with a ruff, cuffs, a loose gown over it, a gauzy apron.'
The doublet is estimated to fetch between pounds 115,000 and pounds 130,000, the portrait - attributed to Paul Van Somer (circa 1577-1622) - is expected to make between pounds 30,000 and pounds 50,000.
The design - in silk, silver-gilt thread and gold lace with motifs that include honeysuckle, carnations, butterflies and snails - has not been copied stitch for stitch by the portraitist. Ms Mayor says it is an artist's interpretation.
Rupert Burgess, head of British pictures at Christie's, said he was waiting for the painting to be delivered from the United States before further researching the picture's attribution, confirming it to Paul van Somer, the portrait-painter from Antwerp who settled in London in 1616.
He said paintings of this period were particularly problematic because until the mid-1620s artists in England rarely, if ever, signed their work. Any records are likely to have been destroyed in the Civil War. He intends to let the National Portrait Gallery 'see it in the flesh'.
The style of doublet was all the rage as an informal dress in the early 17th century.
The original owner was Margaret Layton, of Rawdon in Yorkshire, daughter of a wealthy grocer and prominent citizen of Elizabethan London; she married Francis Layton, a Yorkshireman who eventually became the Master of the Royal Jewels.
Both the portrait and doublet were handed down from one generation to another until 1929, when they were sold at Christie's.
They were bought then for some 4,200 guineas by the Hon Esmond Harmsworth, later 2nd Viscount Rothermere. On 21 June, they will be sold from the estate of Mary, Viscountess Rothermere.
Ms Mayor expects the pair to be purchased by either a museum or a serious collector. No one would dream of wearing the doublet, she said.