Abseiling technicians are cleaning the four dials of the Great Clock at the Houses of Parliament - which houses the bell known as Big Ben - and inspecting for damage.
The experts are working at a height of around 60 metres to clean the clock for the first time since August 2010. The four dials are 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.
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. Clock mechanics will take the opportunity to carry out essential maintenance to the mechanism that moves the clock hands.
Deputy keeper of the clock Steve Jaggs said: "Big Ben is one of the UK's greatest icons, and cleaning the Great Clock is a vital part of its maintenance.
"The process is complex and requires a real head for heights. We have an expert team who will ensure that the clock is thoroughly cleaned and that this piece of our national heritage is safeguarded for future generations."