Under the licensing act (1662) British government agents, called messengers of the press, were made responsible for tracing any form of unauthorized or undesirable printing and reporting its existence, as a prelude to subsequent legal censorship, to a secretary of state. Although the act lapsed officially in 1679, the messengers continued to be used by secretaries of state who required a convenient means of checking on publishers and printers they suspected of sedition.
The question (and answer) were taken from p. 188 of The Encyclopedia of Censorship by Jonathon Green. The book was published by Facts on File in 1990.