Train services running to and from London Paddington may be cancelled or delayed by up to 60 minutes due to a signalling fault.
Operator Great Western Railway says some lines are blocked and disruption is expected until 5pm and tickets are being accepted on alternative routes.
The issue has been caused by a power failure at Paddington affecting signalling, meaning long distance services cannot operate.
The problems come at the same time the M4 westbound is at a standstill due to flooding.
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