A forgotten passage in the British Parliament, which was built for the coronation of King Charles II in 1661, was rediscovered during the renovation, officials said on Wednesday.
It was bricked up in 1807 and has not been accessible for more than 70 years. It was hidden behind a small wooden door that most people assumed contained an electrical cabinet.
The passage led out of Westminster Hall, the only building that survived a devastating fire in 1834 that destroyed the Houses of Commons and Lords.
It was created for King Charles' coronation banquet in 1661, and 17th-century wood still runs over the ceiling.
High-ranking political figures such as diary Samuel Pepys and Britain's first effective prime minister, Robert Walpole, would probably have used it.
"It is incredible to think that this walkway has been used by so many important people over the centuries," said Lower House spokeswoman Lindsay Hoyle.
The passage was reopened during the Parliament's reconstruction after the fire, but closed again in 1851.
Graffiti left by 19th-century bricklayers is preserved on the walls. One section said: "This room was enclosed by Tom Porter, who loved ould ale (beer) very much."
Workers dealing with WWII bomb damage were the next to find the passage, and they installed an electric light and a small access door.
But the wood-paneled door in a cloister, previously used by the Labor Party as an office, had only a tiny brass keyhole and was soon forgotten.
According to Liz Hallam Smith, a historical consultant on the project, historians took a closer look at the work throughout the Parliament building in 2018.
"When we looked closely at the panel, we saw that there was a tiny brass keyhole that nobody had really noticed because we thought it could only be a power cabinet," she said.
"As soon as a key was made for it, the panel opened like a door into this secret entrance."
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