Southern Rail are advising passengers to avoid travelling today as drivers begin more strike action.
Trains will be cancelled today, tomorrow and Friday - with severe disruption also expected on Thursday.
The strike is part of an ongoing dispute over Southern's proposals to increase its use of driver-only trains.
Transport for London staff from three transport unions are on strike this morning.
Backroom staff from the RMT, TSSA and Unite unions are staging a 24 hour walk-out in a dispute over pay and pensions.