The anecdote, discovered by scholars inside the journal of a law student named John Manningham, recounts an amusing story about Shakespeare, his actor friend Richard Burbage and a tryst with a female fan.
In 1602, Burbage was at the height of his fame playing Richard III at the Globe Theatre. After one performance, a female admirer gave Burbage her name and address and invited him to pay her a late-night visit, using the code name "Richard III".
In true Shakespearean comedy style, William overheard this encounter and that evening, when Burbage arrived to call on the woman, the playwright was already on the premises. Shakespeare later sent a witty message to his friend and rival, which stated: "William the Conqueror was before Richard III."
She added: "We think it's true because John Manningham did have mutual friends with Shakespeare such as Ben Johnson and John Donne ... The anecdote was obviously the talk of the town at the time and it fits nicely in with the picture we have of Shakespeare being a witty person."
Burbage and Shakespeare were good friends and the playwright wrote many of his major characters to suit the actor's skills.
The diary entry, dated March 13, 1602, forms part of a wider exhibition which Roly Keating, chief executive of the British Library, said "will seek to cast a new light on how Shakespeare became the cultural icon he is today".
Other items in the exhibition - which runs from April 15 to September 6 next year - include the only surviving play script in Shakespeare's hand, one of only six authentic Shakespeare signatures and rare print editions including the 1623 collection of his plays.
