A mysterious portrait of an unknown woman has been identified as a rare depiction of a young Elizabeth I projecting power, confidence and suitability for marriage.

The discovery was announced by the auction house Bonhams, which said the California owners of the painting had no idea who the sitter was until they had it cleaned. The procedure revealed the picture had been overpainted, probably in the 19th century.

The original subject appeared to be Elizabeth I, a fact confirmed when it was taken to Bonhams this year.

Andrew McKenzie, the director of the auction house’s old masters department, called it an “important and exciting” discovery. “It is really rare to find something this early,” he said.

The painting dates from 1562, four years into Elizabeth’s reign, and is one of the earliest depictions of the monarch, intended to project “a fresh and sophisticated image of her as a youthful monarch radiating a sense of authority and confidence”, said McKenzie.

It comes from a time when getting Elizabeth to marry was high on the English court’s agenda, and courtship was encouraged. Philip II of Spain, the archdukes Ferdinand and Charles II of Austria and Erik XIV of Sweden were all considered possible husbands.

Painting played an important part in the process, with Elizabeth sending her court artist Steven van der Meulen to Sweden to paint Erik.

Circumstantial evidence points to the newly discovered painting coming from the workshop of Van der Meulen, and it is possible its purpose was to excite suitors. “It is highly speculative what it could have been used for,” said McKenzie. “We actually have no idea of where it went between being painted at court in the 16th century and turning up in America this year.”

McKenzie said Elizabeth’s pose was strikingly similar to a portrait of Elizabeth in the Duke of Beaufort’s collection at Badminton House.

“The image of Elizabeth I as the strong all-powerful Virgin Queen is embedded deep in the national psyche but her early years on the throne were dogged with instability and threats from home and abroad,” he said.

“From her accession onwards she and her court were acutely alive to the importance and possibilities of presentation, and this portrait is among the very first we know of to project a new, approved image of the Queen.”

It will be sold in London on 4 December with an estimate of £150,000-£250,000.