Passengers are being told they may not be able to travel at all today due to the closures.
The Brighton to London main line is closed today due to ‘essential’ upgrade work, according to Southern.
All lines between Three Bridges and Brighton/Lewes will be closed.
A spokesman for Southern said: “If your journey is not essential, Southern and Thameslink very strongly advise you to consider travelling on another date.
“The closure of the main line between London and Brighton on what is expected to be a hot day will require the implementation of crowd management measures at stations.
“Your journey will take much longer, you will have to join long queues for trains and buses and there is no guarantee when you will be able to travel.
“Unfortunately, the work taking place cannot be postponed without significant impact to essential major upgrade work on the railway infrastructure.”
Buses are replacing trains between Three Bridges and Brighton/Lewes.
Queues for buses and trains could take hours, Southern said.
The station car park at Three Bridges will be closed to allow replacement buses to operate
Travelling to Brighton to/from stations between Three Bridges and London
A limited train service will operate in both directions between Brighton and London Victoria via Littlehampton with journey times extended by an hour.
This means a journey time of two hours 15 minutes between London and Brighton.
Customers travelling between London and Brighton are advised to use these services rather than the replacement buses
These trains will only operate once an hour and space will be extremely limited.
Not everyone will be able to board these services, the operator said.
If you plan to make a return journey
Ensure you travel early as there will not be space for everyone to board services later in the day.
How is the disruption affecting you? Email your thoughts and pictures to [email protected]