A lamp at the top of Elizabeth Tower - which is switched on in the evening whenever Parliament is sitting - is to stop shining for the first time in more than 70 years.
The Ayrton Light, located above the Great Bell - known as Big Ben - needs to be fully dismantled and restored. A temporary light will replace it.
Installed in 1885, it was previously turned off only during both world wars.
Big Ben will not chime regularly until 2021 because of repairs to the tower.
The light is said to have been installed at the request of Queen Victoria, so that she could see from Buckingham Palace when members of either the Commons or the Lords were sitting after dark.
It is named after Acton Smee Ayrton, a Liberal politician who was First Commissioner of Works between 1869 and 1873.
It is not yet known when the light will switch off, or how long it will be off for.
Big Ben will not be heard from midday on Monday. The House of Commons has said it will look again at the length of time it will be silenced after "concerns".
Parliament said it had to protect workers carrying out the renovations.
But Prime Minister Theresa May said "it can't be right" that the bell will not chime regularly again for four years.
It will still sound for important events including New Year's Eve and Remembrance Sunday.
Big Ben basics
- The Great Bell forms part of the Great Clock in the Elizabeth Tower - commonly known as Big Ben
- It weighs 13.7 tonnes and the Elizabeth Tower stands 96m (315ft) tall
- Every hour it strikes an E note, and every 15 minutes four "quarter bells" chime
- To stop the chimes, the striking hammers will be locked until 2021