LONDON — A 360-year-old passageway once used by British monarchs has been rediscovered inside Parliament, revealing a piece of history that was thought to have been permanently covered up after World War II.

For decades, it was thought that the only remnant of the passageway, built in 1660, was a brass plaque inside Westminster Hall, the oldest of Parliament’s buildings, marking where the entrance would have been.

Instead, access to the passage had remained hidden in plain sight for about 70 years.

“To say we were surprised is an understatement,” Mark Collins, historian of the Parliament’s estates, said in a statement on Wednesday. “We really thought it had been walled up forever after the war.”

The passage, created for a procession to the 17th-century coronation banquet of Charles II, was then used for about 150 years for other coronations and by lawmakers to gain access from the hall through to the original House of Commons chamber. Among the luminaries likely to have used the route include, historians believe, Robert Walpole, who is generally regarded as Britain’s first prime minister.