First class travel is being scrapped on all Greater Anglia trains except intercity services from January 2020, the firm has announced.
The rail operator said it was also bringing in new longer trains from the end of next year.
Both measures would provide 20% more standard seats, it said.
Martin Moran, Greater Anglia's customer services director, said the plans aimed to "reduce overcrowding and improve our customers' experience".
Only the new intercity trains, which run between Norwich, Ipswich, Colchester and London, will have first class seating.
The company has written to all first class annual season ticket holders to inform them of the new policy, with details of how to swap a first class ticket for a standard ticket.
Key services where only standard seating will be available from January 2020 will be:
- Clacton to London
- Suburban services from Manningtree/Colchester to London - intercity services will still offer First Class, Braintree/Witham to London
- Chelmsford to London - hourly off peak intercity services will offer first class, Southend Victoria to London
- Cambridge to London via Bishops Stortford
- Stansted Airport to London
- Harlow to London
- Hertford East to London
- 12 September 2017
- 6 July 2017
- 7 June 2017