The railway company said on its Twitter at 11.37am that there is a points failure at Wivelsfield.
“If you’re travelling between Brighton and Haywards Heath, you should delay travelling until later if you can,” said a spokesperson.
“If you have to travel now, your journey will take a lot longer and you will need to use an alternative,” they added.
Southern said commuters can use their train tickets on Metrobus services between Brighton and Three Bridges, as well as Brighton & Hove buses on any route.
People can also find alternative route maps here.