Parliament's Great Clock, also known as Big Ben, is being given an essential clean-up. Abseiling technicians are scrubbing each of the four faces of the clock and checking them for damage, working at a height of around 60 metres.
One week has been set aside for the delicate process allowing one day for each clock dial, and contingency in case of poor weather conditions.
Last cleaned in 2010, the Great Clock has four dials, each made up of 312 pieces of pot opal glass, held together by a cast iron framework.
The clock hands are made of hollow copper sheet, which replaced the original cast iron hands when they proved too heavy.
As well as removing any dirt that has built up since 2010, the technicians will carry out a photographic survey to check the clock dials for damage.
The Great Clock will continue to measure time throughout the cleaning process, but the hands of the clock will be temporarily paused as each side is washed.
The Great Clock, which tops the Elizabeth Tower, began keeping time on 31 May 1859. The ornate design of the Tower and the dials of the clock were created by Charles Barry and Augustus Welby Pugin.
The clock mechanism itself was designed by Edmund Beckett Denison, a lawyer with an interest in clock making.
It is part of a grade 1 listed building and UNESCO World Heritage site.