Thousands of commuters have been affected by major signalling problems this morning on the Greater Anglia line.
Greater Anglia is advising people not to travel while the problem is being fixed.
The signalling problems are in the Chelmsford area, disrupting services between London Liverpool Street and Norwich.
Customers have been told they can use their tickets from West Anglia stations, such as Cambridge, Stansted Airport and Bishop's Stortford. Tickets will also be valid on First Capital Connect services from Cambridge to Kings Cross and on London Underground services.
A limited bus service is in operation between Witham and Stansted Airport
Network Rail engineers are working as fast as possible to fix the problem.