The Westminster light which shines to show Parliament is sitting is to be switched off for the first time in more than 70 years.
The Ayrton Light, housed in the Queen Elizabeth Tower above the Great Bell, Big Ben, is set to be removed for repairs.
It comes as the famous Great Bell is set to fall silent on Monday for restoration work.
The light was installed in 1885, to be switched on at night so Queen Victoria could see from Buckingham Palace when Parliament was sitting.
It is named after Acton Smee Ayrton, the first Commissioner of Works at Parliament.
The Ayrton Light will be replaced with another lamp while it is being repaired, but it is not known how long the repairs will take.
Big Ben will fall silent from midday on Monday, although it will still sound for major events including New Year's Eve and Remembrance Sunday.
It was due to be silent for four years, to protect workers carrying out renovations to the tower from noise, but Prime Minster Theresa May has asked for this to be reviewed.