Two lost works by the Italian master Caravaggio discovered in the Queen's collection are to be shown for the first time since they were identified.
The painting was acquired by Charles I in 1637
The Calling of Saints Peter and Andrew and A Boy Peeling Fruit had been regarded as copies and largely ignored until re-examined by researchers.
It has taken six years of study for their authenticity to be confirmed.
The Art of Italy exhibition opens at the Queen's Gallery at Buckingham Palace on 30 March.
The Calling of Saints Peter and Andrew sat in a storeroom at Hampton Court for decades becoming obscured by varnish and dirt.
Desmond Shawe-Taylor, surveyor of the Queen's pictures, said: "This is a huge addition to the collection."
Discovery that this piece was an original led to the confirmation of the simpler piece A Boy Peeling Fruit.
Michelangelo Merisi da Caravaggio, a notorious brawler who fled from Rome after killing a rival in a duel, was born in 1571 and was found dead on a beach in 1610.
He is one of the most highly regarded Italian masters, whose biblical scenes with their mastery of light and darkness are considered revolutionary. Only 50 or so of his canvases are known to exist.
The exhibition will be the first of Italian art from the Royal Collection for more than 40 years.
It brings together 90 paintings and 85 drawings from royal palaces and residences.