Rail commuters are facing a morning of travel misery after signalling equipment was damaged near Slough during overnight engineering work.
First Great Western (FGW) is operating a "severely reduced" service into Paddington, adding up to an hour to journeys.
The rail company has advised passengers to avoid travelling between Reading and Paddington.
It said the disruption would continue until at least midday.
A message on the FGW website said: "Following overnight engineering work, signalling equipment has been damaged between Slough and West Drayton affecting all four lines. Because of this, a severely disrupted train service is operating between Reading and London Paddington this morning.
"Many services will be cancelled and there will be delays of up to 60 minutes affecting those trains which can run. We strongly advise customers to avoid travelling where possible. Disruption is expected to last until at least 12:00 today.
"At present, we are expecting to be able to run four trains per hour in each direction between Reading and Paddington. Your tickets will be valid on other operators' services and on alternative routes which avoid the affected area."
