Many Hampton Court Palace visitors barely noticed the Caravaggio

The painting - The Calling Of Saints Peter And Andrew - depicts the two saints with a young, beardless Christ.

It was in a minor room at Hampton Court Palace, Surrey, but was long overlooked because of centuries of built-up dirt.

Leading British art connoisseur Sir Denis Mahon has said he now has little doubt it is an original Caravaggio.

Royal collector

That would boost its value from �100,000 to tens of millions of pounds, according to a report in the Times, although it adds that it is almost unthinkable the work would ever be sold.

The Calling Of Saints Peter And Andrew was bought by King Charles I from an art dealer in 1637.

Charles I was an avid art buyer and, during his reign from 1625 to 1649, built up a collection including works by Raphael, Titian, Corregio, Mantegna, and other ancient and Renaissance paintings and sculpture.

But in recent years, many art handbooks and guides had tentatively listed it as a likely copy of the painting by the Italian, who has most of his work displayed in his home country.

THE LIFE OF CARAVAGGIO Italian artist 1573 - 1610 Worked in Rome, Naples and Sicily Painted religious works and everyday scenes Renowned for use of light and shadow Asked to produce works for Rome's Contarelli Chapel before he was 30 Fled Rome when accused of killing an opponent during a tennis game Pardoned of murder by the Pope, three days after his death

It has been largely ignored by the visitors - including many art scholars studying the art on display - to the palace.

But now Sir Denis is convinced the painting in a side room at the palace should actually have been the focus of their visits.

He is a leading expert on 17th Century Old Masters and has an extraordinary track record in rediscovering lost paintings.

In many cases, he has removed layers of varnish added to paintings over the decades and unearthed evidence that they were indeed original versions of the works by some of history's most famous artists.

As a trustee of the National Gallery in London from 1957 to 1973, he oversaw the acquisition of works by Poussin, Tiepolo, and Degas.

His verdict on the painting - which has prompted the Royal Collection to carry out further restoration work to confirm it - suggests the Queen now has her first Caravaggio in her extensive art collection.