The chairman of the Shakespeare Birthplace Trust made a robust defence yesterday of a recently-discovered painting which he is “90 per cent sure” is the only lifetime portrait of William Shakespeare.

The claims were dismissed as “codswallop” by Sir Roy Strong, former director of the Victoria & Albert Museum and the National Portrait Gallery, at the weekend.

Sir Roy told a Sunday newspaper Prof Stanley Wells had “gone off on a fantasy journey”.

The outspokenness of Sir Roy’s intervention on a fellow academic is made even more remarkable by the fact that he is an honorary fellow of the Shakespeare Birthplace Trust.

The controversial painting, discovered in the Cobbe family collection in a country house in Ireland, goes on display for the first time today at the centre of a small exhibition, Shakespeare Found, at the Shakespeare Birthplace in Stratford-upon-Avon.

Asked about Sir Roy’s remarks, Prof Wells said: “I thought it was pretty disgraceful. I believe he was talking of the cuff but he’s a big boy, he ought to know that when you are talking to a journalist you need to be careful what you say.”

A statement issued at the exhibition launch challenges Sir Roy’s belief that the true subject of the Cobbe portrait is the poet Sir Thomas Overbury. Supporters of the Shakespeare attribution point out flaws in the research of the late art historian David Piper, which identified Overbury as the sitter. They also object that the features in the Cobbe painting do not match those in a known portrait of Overbury.

Prof Wells added that he felt “let down” by the organisers of an exhibition of 18th century depictions of Shakespeare, just opened in London, who dismissed claims for the new portrait despite having been lent several items from the Shakespeare Birthplace.

“I feel that a lot of the opposition is a knee-jerk scepticism,” he said. “It’s the duty of scholars to be sceptical, but it’s also the duty of scholars to examine the evidence. I don’t feel that anybody who has commented on it has done that so far.

“Over the last three years I hope I have been doing what I think other scholars ought to do, which is to listen to the evidence.

“I have said myself that I wish we had documentary evidence. We are not in a position to say with 100 per cent certainty, but on the other hand I think the evidence is strong enough to make it 90 per cent.”

The painting came to light after Alex Cobbe, a picture restorer who worked at Birmingham Museum & Art Gallery in the 1970s and is a member of the family which owns the painting, noticed its resemblance to a supposed portrait of Shakespeare shown in an exhibition at the National Portrait Gallery. Subsequent research has built up the case that the Cobbe painting, once thought to be of Sir Walter Raleigh, is the original from which four copies – each of which has been independently claimed to be a portrait of Shakespeare – were made.

Two of the copies are included in the exhibition, while another has slipped from public view since it was sold in 1947. The fourth is the well-known Folger portrait in Washington DC.

Circumstantial evidence supporting the claims for the portrait centres on the fact that the Cobbe family is linked to that of Henry Wrothiesley, the Earl of Southampton, who was Shakespeare’s patron. Another painting in the family collection, previously thought to be of a woman, is now generally accepted as showing the effeminate young Wrothiesley.

This painting is also included in the exhibition, along with a copy of a later portrait of Wrothiesley when he was imprisoned in The Tower of London for his part in the Earl of Essex’s rebellion.

n Shakespeare Found is at Shakespeare’s Birthplace, Stratford-upon-Avon, until September 6.