Engineering work is taking place between Romford and Shenfield, closing some lines.
All day on Saturday and before 09:00 on Sunday, trains between Romford and Shenfield will be retimed, with extended journey times by up to 15 minutes.
Trains from London Liverpool Street will be retimed to run up to 15 minutes earlier than usual.
Check before you travel:
You can plan your journey using the National Rail Enquiries Journey Planner