The only known authentic likenesses of the Bard are the familiar ones of a round-headed bald man that exist in the First Folio of his collected works and the effigy on his monument at Holy Trinity Church in Stratford-upon-Avon. Both were made posthumously.

Over the years there have been many claims to have found the real Shakespeare, the most recent being in 2009, when Shakespeare Birthplace Trust suggested that a painting, known as the , could be the only portrait of Shakespeare painted in his lifetime. Academics then queued up to pour scorn on the claim.

Griffiths, with a substantial amount of compelling evidence, is claiming that this is the living face of a 33-year-old Shakespeare at the height of his celebrity - shortly after writing and shortly before .

The engraving by William Rogers, England's first great exponent of copperplate engraving, is on the title page of a groundbreaking 1598 book called or by horticulturist John Gerard (1545-1612). It is full of elaborate decorative devices, flowers and symbols which surround four male figures, who had been generally assumed to be allegorical.

Griffiths, in the course of writing a book about Gerard, tasked himself with discovering who the men might be. To do this, he had to crack an elaborate Tudor code of rebuses, ciphers, heraldic motifs and symbolic flowers, which were all clues pointing to who the men were.

The relatively easy ones were Gerard himself, Flemish botanist Rembert Dodoens, and Queen Elizabeth's chief minister and closest adviser Lord Burghley, who was Gerard's patron. So that left the tricky fourth man, bottom right.

"My problems began," Griffiths says. "He's dressed as a Roman, wearing laurels and meant to make us think of Apollo and poetry … I couldn't think of anybody really who was a direct intimate of Gerard's and was involved in writing his book."

Slowly, after reading up on how devious the Elizabethans were in encoding their meanings, Griffiths began, he says, to solve the clues provided by the symbols and flowers around the fourth man.

They include a figure "4" and an arrow head with an E stuck to it. In Elizabethan times people would have used the Latin word "quater as a slang term for a four in dice and cards. Put an e on it and it becomes quatere, which is the infinitive of the Latin verb quatior, meaning shake. Look closely and the four can be seen as a spear," Griffits says.

"It is a very beautiful example of the kind of device that Elizabethans, particularly the courtiers, had great fun creating."

Underneath it is a W - for William? And OR? A few months before it was engraved Shakespeare's father was granted a coat of arms with a golden background. The heraldic term for gold is OR.

Griffiths believes the many clues lead to it being Shakespeare, that he has not a shred of doubt. "For me it is not about doubt or supposition. I'm faced with a series of facts that I can't gainsay, as much as I try. This is what these facts are, these are what the plants are, this is what they signify, this is what the symbol decodes as. All of that adds up to Shakespeare. I can't make that - and believe me I've tried - add up to anybody else but Shakespeare."

All of which raises the question: why is he there?

Griffiths believes Shakespeare was given his literary start by Burghley, the most powerful man in the country - and that he became almost a political propagandist for him. If you accept that theory, then Shakespeare, of course, would have moved in the same circle as Gerard as both men had Burghley to thank for their careers.

Griffiths says his theory is that Shakespeare helped Gerard with Greek and Latin translations in the book, and acted as a kind of script doctor. So the four men are the writer himself, his patron, his inspiration (Dodoens) and his literary adviser.

But Griffiths promises more to come on that score, including the discovery of a new short play, which he will claim is by Shakespeare.

Griffiths says he has consulted an eminent but anonymous Shakespearean scholar, who had approved of his method. He also enlisted the help of Edward Wilson, emeritus fellow of Worcester College, Oxford, who set about trying to disprove it being Shakespeare.

Wilson says he failed. "We do not think anyone is going to disprove it," he says.

"This is the most important contribution to be made to our knowledge of Shakespeare in generations."

Shakespearean studies is littered with examples of scholars finding a "true" image of Shakespeare only for it to be proved false. Just as Griffiths claims he has cracked a code there will be academics examining his evidence for flaws.

Professor Michael Dobson, director of the Shakespeare Institute at the University of Birmingham, is among the first to pour scorn on the theory. "I'm deeply unconvinced," he says.

"I haven't seen the detailed arguments but is not the first publication to make this sort of claim.

"One has seen so many claims on Shakespeare based on somebody claiming to crack a code. And nobody else has apparently been able to decipher this for 400 years.

"And there's no evidence that anybody thought that this was Shakespeare at the time."

He adds: "I can't imagine any reason why Shakespeare would be in a botany textbook. It's a lovely picture. Everybody is very fond of it. But that doesn't mean that he had anything to do with it apart from the fact that he read it.

"It's a man in a toga, holding a little bit of a corn on the cob in one hand and a fritillary in the other."