Following inspections of the track between Paddington and Maidenhead, Network Rail has confirmed that floodwater is likely to affect trackside signalling and safety equipment this morning.
While they work to fix the problem, First Great Western (FGW) will only be able to run four trains an hour between Paddington and Reading in each direction.
This is around a fifth of the services that would normally be scheduled.
This will continue until Network Rail lets FGW us know it is safe to run a full service again.
FGW strongly advise customers not to travel unless it is absolutely necessary, and to consider alternative routes.
In recognition of the disruption expected, and to allow customers to postpone their journeys until later in the week, ticket restrictions have been lifted across the FGW network until Friday.
During the day, services in the following areas are expected to be affected as outlined below. Additional trains will run during peak times :
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