Unknown Rubens that could be a £6m find

A painting by the Flemish Old Master Sir Peter Paul Rubens that was unknown until it turned up for auction in London is expected to fetch up to £6 million next week.

The only information about the history of The Portrait of a Young Woman is what experts have been able to glean from seals and other information on the back.

It was painted in the early years of the artist's time in Italy, which included a short diplomatic mission to Spain, from around 1600 when he was 23.

It was a formative period and this work may have been part of a series commissioned by his patron, Vincenzo Gonzaga, the Duke of Mantua, of ladies at the Spanish court.

But experts were unaware of the work until its British owner, who bought it 25 to 30 years ago, took it to Sotheby's and asked whether it was by Rubens.

A seal from the period 1807 to 1814 showed it was at one stage in the hands of the Accademia — the equivalent of the Royal Academy — in Venice.

It is thought likely it stayed in Italy during the early 19th century before entering the collection of the Hanmer family in Britain between 1830 and 1850. The family, then headed by Sir John Hanmer at Bettisfield Park in Cheshire, were known art collectors. The Hanmer name is on the back of the canvas.

It is thought the family probably sold it in the Thirties or Forties leaving another half century of missing history.Alex Bell, Sotheby's Old Master expert, said: "It's rare to get a good Rubens portrait. She's a pretty woman, beautifully painted.

It's an unfinished picture. You can see the process of creating a work of art where you concentrate on the bits that matter — face and hands — first."

It will be sold next Wednesday with an estimate of £4 to £6 million.