Passengers who normally make arrangements to 'meet under the clock' at Waterloo will have to change their plans during March and April while the historic timepiece is restored.
From Tuesday 9 March specialist clock maker Smith of Derby will remove the hands and many of the internal workings of the clock so they can be re-engineered to work with the latest technology.
The upgraded equipment will sit behind the clock's historic exterior which will also be cleaned and decorated as part of the project.
"The main concourse clock has been a central feature of Waterloo Station since the early part of the 20th century," says Richard O'Brien from Network Rail.
"It has been more than 20 years since the last major overhaul and many of the internal workings are now obsolete. We now have the opportunity to install the most efficient power and latest timekeeping technology so the clock can remain an important characteristic of the station for many years to come.
"There are lots of other clocks within the station, including on all the departure boards, so people will still be able to find out the time. However, if passengers are planning to meet family, friends or business associates we suggest arranging an alternative meeting place while the work is carried out."
The clock was made by Gents of Leicester and is believed to date back to the 1920s.
The refurbishment is scheduled for completion by 20 April. The majority of work will be carried out off-site and during this time the main clock fitting will remain covered up.
At the same time the Victory Arch clock – dedicated to Southern Railway employees killed in World War I – will be serviced and some of the internal workings replaced.