Rail passengers have complained of delays and cancellations to services on the first weekday following significant timetable changes.
GWR said a signal fault between Burnham and Slough had affected services.
A new, super-fast GWR service from Bristol to London, due to leave at 08:53 GMT, was among the cancellations.
Under the new timetable non-stop trains between the two cities have begun operating, reducing the journey time from Temple Meads by 17 minutes.
Many other services from the west of England to London were delayed.
One of the biggest delays was the 04:51 GMT service from Plymouth, which arrived at London Paddington 42 minutes late, while the 05:34 GMT and the 05:47 GMT were also heavily delayed.
Services in other parts of the country were affected as new timetables came into effect, including the cancellation of some operated by Northern due to a shortage of drivers and delays, and cancellations on Transpennine Express (TPE) services.
'Chaos on lines'
One passenger, Ollie Willis, wrote on Twitter it was a "stupid timetable" and said he was on a smaller train with less space.
Terry Harrison tweeted to say the timetable change had caused "chaos on the lines".
Not a good start to @GWRHelp new super improved speeded up timetable this morning. The very first non-stop Paddington to Bristol Parkway journey at 09:15..... cancelled 🙄🤦♂️ pic.twitter.com/qSO50jb9HL— BusAndTrain User (@BusAndTrainUser) December 16, 2019
The new timetables were advertised as the "biggest timetable change on the network since 1976" and would be offering "faster journey times, more seats, and more frequent services".
A GWR spokesman said a "signal system fault" on the main line between Burnham and Slough had led to the delays, and fewer trains were able to run.
The fault has now been fixed but services may continue to be delayed or cancelled until things get back to normal.