ALEC COBBE was strolling around the Searching for Shakespeare exhibition at the National Portrait Gallery when he was stopped in his tracks by a painting that was the spitting image of one he had on his wall at home.

It had been in his family’s collection for centuries and no one had paid it much attention, although an 18th-century ancestor thought that it might have depicted Sir Walter Raleigh.

Scholars have confirmed that Mr Cobbe’s painting is the original of the famous portrait in the Folger Shakespeare Library in Washington that was on loan at the National Portrait Gallery exhibition — an image that inspired numerous copies in the 18th and 19th centuries, fixing it in the public imagination as an image of Shakespeare.

While research suggests a date of 1610, six years before Shakespeare’s death, what makes the discovery particularly exciting is that it belonged to the third Earl of Southampton, Shakespeare’s patron and, some have argued, the “fair youth” of the sonnets.

Stanley Wells, the chairman of the Shakespeare Birthplace Trust and one of Britain’s leading experts in the field, told The Times: “This is a very interesting find. Its emergence in a collection which belonged to Shakespeare’s patron is in itself of considerable interest. It’s not impossible that it’s Shakespeare.”

Mr Cobbe, a paintings restorer, told The Times that he had not given much thought to his painting until he saw “its mate”. It will be given pride of place within the important Cobbe collection of art and furniture housed at Hatchlands Park, a National Trust property at East Clandon, near Guildford, Surrey.

The discovery comes four years after another portrait in the Cobbe collection made headlines. An effeminate appearance had previously led to its identification as a lady, but research revealed that it was the earliest known oil portrait of the Earl of Southampton.

The Cobbe pictures were brought to Ireland from Hampshire in 1717 by Charles Cobbe, later Archbishop of Dublin, and almost exclusively came from his Norton inheritance. His father and uncles had been heirs to the childless Nortons, one of whom, Lady Elizabeth Norton, was the 3rd Earl of Southampton’s great-granddaughter.

There is, however, no painting, drawing or sculpture that anyone can say with any certainty is a true likeness of Shakespeare or, indeed, that was made by anyone who knew the playwright. The National Portrait Gallery exhibition — which has transferred to the Yale Centre for British Art in New Haven, Connecticut — brought together for the first time eight of the more celebrated portraits traditionally identified as Shakespeare.

The public’s image of the playwright — with his domed forehead and neatly clipped beard and moustache — is based primarily on the Chandos, dating from between 1600 and 1610 and so called because it was once owned by the Dukes of Chandos — and the engraving by Martin Droeshout printed on the title page of the First Folio of Shakespeare’s works published in 1623. After the Chandos portrait, in the National Portrait Gallery’s own collection, the Folger has the longest tradition of being identified as Shakespeare.

From its emergence in the 18th century three key elements made the Folger painting a compelling contender as a portrait of Shakespeare from life. While the sitter’s age and the date, inscribed “AEte 46 / 1610”, accorded with Shakespeare’s birth in 1564, the sitter’s high forehead matched the essential iconography provided by the Droeshout portrait and the Stratford memorial bust, and the lace-trimmed collar and rich doublet were consistent with the 1610 date.

In 1988, however, doubts about the sitter’s identity emerged after conservation work proved that the original hairline and much of the hair had been overpainted and that the inscription had been placed on top of this overpainting.

The alteration was reversed to reveal its present state — one that corresponds with the Cobbe portrait.