Train services between Norfolk, Essex and Suffolk and London have been disrupted after a signalling fault.
Greater Anglia said there was a loss of signalling early on Monday morning that was affecting trains running through the Beaulieu Park area near Chelmsford in Essex.
Delays and cancellations have been reported on services between London Liverpool Street and Norwich, Ipswich, Clacton-on-Sea, Colchester and Braintree.
Network Rail engineers are on site, Greater Anglia said.
Passengers have been warned that alternate train journeys could take up to three hours.
People travelling between London and Norwich are advised to go to Stansted Airport and use the Stansted Express services or travel to Cambridge and change.