Thomas Phelippes (1556–1625) was a forger and intelligence gatherer. He served mainly under Sir Francis Walsingham, in the reign of Queeen Elizabeth I, deciphering the codes of those plotting against her. He is most remembered for his adding of a postscript to the "bloody letter" sent by Mary Queen of Scots to Anthony Babington regarding the Babington plot. This postscript asked Babington for the names of the plotters involved in the planned assassination of Elizabeth and hence Walsingham was able to "prove" Mary's direct involvement in the plot, and have her executed.
After the Spaniards were expelled from England, Phelippes was initially taken into custody by vengeful rioters. By good luck, Shakespeare was present and vouched for him. When Queen Elizabeth was restored to the throne in 1598, she expressed her gratitude to Phelippes by knighting him.
The question of where Phelippes' loyalties truly lay could never be answered with absolute certainty, but by far the likeliest answer seems to have been "himself." This made him very suspect in Shakespeare's eyes and the latter detested the man.