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The picture was identified after Sotheby’s Amsterdam emailed a photograph of the 4ft 7in by 6ft (1.4 x 1.83m) to Bond Street Old Master specialist George Gordon.

When the picture, which depicts the slaughter of newborn boys on the orders of King Herod, was bought in 1920, by the father of the current owner, it was attributed to Jan van den Hoecke a pupil and assistant to Rubens; until now its authorship had never been questioned.

Sotheby’s are basing their Rubens attribution on a theory that the picture, which was once in the collection of Prince Liechstenstein of Vienna, was miscatalogued in the 18th century when the Liechtenstein inventories were compiled. Records show that when Prince Liechstenstein bought the picture it was on the understanding that it was an early Rubens, and The Massacre of the Innocents still bears the Liechstenstein family seal.

The last Rubens of such importance to be offered for sale in London was Samson and Delilah, also from the Liechtenstein collection, which was bought by the National Gallery at Christie’s in July 1980 for £2.53m.

The painting, which is thought to have been painted in Antwerp between 1609 and 1611 following Rubens’ sojourn to Italy, is said to be in extraordinary condition for a picture of its age, with both the painted surface and the wooden panel remaining almost perfect.

When the picture is offered in July it will be with an estimate of £4-6m, but it could to make as much as £20m.