A recently rediscovered painting by Hans Holbein is expected to fetch between £2m and £3m at auction.
Research has continued since the painting resurfaced in 1974
The Portrait of Sir Thomas Wyatt the Younger, painted in about 1541, will go on sale at Sotheby's in London in July.
Its identity is thought to have been unknown for more than 200 years, before it was confirmed earlier this year.
Sir Thomas was executed for treason in 1554 after leading a rebellion against the projected marriage of Mary I to Philip II of Spain.
The circular - or tondo - portrait is thought to have remained in the Wyatt family for almost 200 years before its identity was lost.
It resurfaced in 1974, when it was sold at Christie's as an anonymous British school portrait.
The then-director of the Victoria and Albert Museum, Sir Roy Strong, compared it at that time with three other versions of the portrait.
Subsequent research was undertaken by Sir Roy, and by Bodo Brinkmann at Stadel Museum in Frankfurt.
Sir Roy confirmed the painting as the lost original by Holbein in Apollo magazine earlier this year.
"In terms of English portraiture this picture is unprecedented, a major landmark in every way in the reception of the new Renaissance ideals at the court of Henry VIII," he wrote.
Hans Holbein the Younger was born in Germany in 1497 or 1498 and moved to England in his late 20s, where he became a court painter.