A broken down train at Battersea Park is causing rush hour chaos at Victoria Station.
There are delays of at least 45 minutes on Southern trains running from Victoria through Battersea Park which are expected to last until at least 5pm.
Gatwick Express services are also affected, with trains running only every 30 minutes and additionally stopping at East Croydon and Clapham.
Passengers have been advised that tickets will be accepted on South West, Southeastern and Thameslink services as well as on the Underground and buses.
The disruption follows major chaos at Kings Cross station during today's morning rush hour and severe delays on the Hammersmith & City, District, Circle and Metropolitan lines.
The RMT union today slammed Boris Johnson as an "absentee mayor" and called for a travel summit to be held in the wake of the chaos. Yesterday evening Thameslink service were disrupted by overhead line problems and Overground trains were also delayed this morning.
“This has been yet another grim 24 hours for London commuters," said RMT boss Mick Cash.
"While the Mayor is parading about Manchester pumping up his Tory leadership campaign the city he is supposed to run is sliding deeper and deeper into transport chaos. RMT has warned repeatedly that cuts to staffing, budgets and maintenance on both tube and rail has left services permanently on a knife edge and last night and this morning we have seen what that means in practice. RMT shares the anger and frustration of passengers caught up in this shambles and the union will continue to fight for a reversal of the cuts programme which threatens public safety as demand continues to surge.”