Rail commuters are facing 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 company has advised against travel between Reading and Paddington.
Network Rail said some lines have reopened as work to repair the problem continued.
The problems are also affecting services to and from the west of England, Swansea, Bristol, Oxford, Cheltenham Spa and Worcester.
The company said it "sincerely apologised" for the problems and said tickets would be valid on other operators' services and on alternative routes which avoid the affected area, as well as for train journeys on Tuesday.
Network Rail said some lines have reopened although "severe delays" can still be expected until the end of service on Monday.
It said it was confident a full service would resume on Tuesday.
More than 1,600m of cables were replaced as part of the efforts to repair the damage to the signalling system.
MP for Reading East Rob Wilson tweeted: "It's not acceptable to have signalling crisis once a month on 1 of Europe's busiest train routes."