The upgrades in south London will improve reliability on the network while major work to improve accessibility at Gatwick Airport station will make journeys easier for all passengers and support people with reduced mobility.
The investment to modernise 1980s track and signalling will help even more passenger and freight trains arrive at their destinations on time.
Over May and June, the track, switches and crossing at Falcon Junction, near Clapham, and Balham Junction will be renewed and the track drainage in the area improved.
Preparation works for the new signalling will continue including laying new signalling cables and constructing the foundations for new signal gantries between Balham and Clapham.
This phase of work, to take place between London Victoria Clapham Junction and Balham, and the West London Lines from Clapham to Shepherds Bush, will continue until after Christmas 2022.
On the following dates, no trains will run between Victoria and East Croydon, with most being diverted to London Bridge:
Saturday 1 to Sunday, May 3 (Bank Holiday)
Saturday 8 to Sunday, May 9
Saturday 15 to Sunday, May 16
Saturday 22 to Sunday, May 23
Saturday 29 to Monday, May 31 (Bank Holiday)
Sunday, June 6
Sunday, June 27
Rail tickets will be accepted on London Underground services between London Victoria, London Bridge and Balham, and on London Buses between London Victoria and Clapham Junction and between Balham and Streatham Hill.
Tickets will also be valid on South Western Railway services between Clapham Junction and London Waterloo.
A limited replacement bus service will operate between East Croydon and Clapham Junction for local journeys.
Work on improving accessibility around the Gatwick Airport station will also continue with track realignment and platform strengthening activity, which will allow wider platforms, more lifts, escalators and stairs to be installed.
Shaun King, route director for Sussex said: “The work we’re undertaking over the May Bank Holiday and beyond, will result in passengers enjoying even more reliable and comfortable journeys in the future.
"I’d like to thank passengers for their patience while we carry out our work and remind them to check their journeys in advance to see how they may be affected.”
Chris Fowler, customer services director for Southern, added: “We’ve been working closely with Network Rail and other transport operators to make sure our customers can get where you need to be during this vital modernisation programme.
"As more people return to the railway, we’d advise everyone to use our online tools to plan your route and check the busiest services before you travel.
"Please travel at a quieter time if you can.
“You can rest assured that we’re working round the clock to keep trains and stations clean and safe - please remember you still need to wash your hands often, wear a face covering and keep your distance.”