By Stephen Dowling
BBC News Online entertainment staff
On Thursday, The De Vere Society marked the 400th anniversary of the death of a man it believes wrote Shakespeare's plays.
Many of the author's rivals have been called 'the real Shakespeare'
Its members believe an Elizabethan nobleman, Edward de Vere, really wrote the many plays, poems and sonnets attributed to William Shakespeare - works that transformed the English language.
Shakespeare scholars say the society's doubts fly in the face of established evidence. But they are not the first to question whether the Stratford-born writer was the real author.
Over the years, conspiracy theorists have come up with a number of possible authors who may have used William Shakespeare as a pen-name, including some of his most respected contemporaries.
Francis Bacon, the writer and philosopher, has been the subject of one of the most long-running arguments in English literature - the Baconian controversy - which still has its followers.
Likewise, the playwright and poet Christopher Marlowe has also emerged as one of the leading names in the great debate.
Other figures put forward as the "real" Shakespeare have included writer Ben Jonson, the nobleman Sir Walter Raleigh and even Queen Elizabeth I.
The theory about Edward de Vere, a noted literary patron in the court of Elizabeth I, started in 1920, after the publication of a book about him by an English teacher called J Thomas Looney.
Many of the doubts over Shakespeare's authorship were prompted by the Bard's background - his father was an illiterate butcher and his hometown, Stratford-upon-Avon, was considered a provincial backwater.
"Looney suddenly realised it was impossible for Shakespeare to have written all the plays because of his family background, and lack of opportunities," Richard Malim, general secretary of The De Vere Society, told BBC News Online.
Mr Looney studied writers of Shakespeare's time and came up with 14 tests, such as whether the writer was a Catholic sympathiser, to find the possible author. He decided de Vere was the writer because a poem showed similar traits to Shakespeare's work.
Mr Malim also said research showed Shakespeare was "too young" to have written his first play, Love's Labours Lost, which he claims was written in a style at least 12 years out of date when first performed in 1594.
Michael Wood said details of Shakespeare's private life are scarce
But why would de Vere hide his identity? Mr Malim argues it was not deemed suitable for a man of his standing to be seen writing plays for public consumption, so he kept it a secret - even though he died nine years before Shakespeare's last work was unveiled in 1613.
Michael Wood, a producer and programme-maker who presented the BBC series In Search of Shakespeare, believes the theories about "secret Shakespeares" have no basis in fact.
But he did say so many theories had been prompted by the frustrating lack of detail about the playwright's life.
"There is this problem with his biography that always occupies people," he told BBC News Online.
But he said "no professional historian who had studied the primary sources has ever thought Shakespeare did not write the plays".
"For somebody so famous, we know very little about his interior life," Mr Wood said. "He was very guarded and reticent about life.
"We all want to know more about him and he won't let us."
Mr Wood said Shakespeare did not come from an educated family - but his father had become mayor of Stratford and had paid for his son to be educated at Stratford Grammar School.
Mr Wood also said Shakespeare was writing at "a time of acute tensions" in England, with the state religion veering from Protestant to Catholic and allegiances constantly changing - and this alone may have been the cause of the author's reticence.