Major disruption to rail services caused by safety checks into cracks on some carriages could last for days, operators have warned.
Passengers on west of England and south Wales services run by GWR and East Coast main line services run by LNER were advised not to travel due to the disruption.
Hull Trains and TransPennine Express (TPE) between Newcastle and Liverpool were also affected.
The problem stems from concerns about jacking points on the fleets of new Class 800 Hitachi high-speed trains.
Hitachi Rail said in a statement that it took the decision to carry out precautionary checks after routine inspections revealed cracks on some carriages.
GWR, which operates 93 Hitachi 800 trains, has warned the disruption to services is likely to continue “into the following week” and urged customers to request refunds.
“Train timetables are presently being worked on and will be published as soon as possible.”
They added the issue is affecting GWR journeys between Paddington and Bristol, Swansea, Penzance, Hereford and Cheltenham Spa, and that other services still running will be “very busy” as a result.
Robert Nisbet, regional director the Rail Delivery Group, told BBC Breakfast on Sunday: “With the inspections completed initially by the end of today, we are still expecting some disruption to carry on for a few days.
“It’s impossible for me to say exactly how long that is going to take, but we are obviously going through this as quickly as possible, but we don’t want to rush it.
“We want to ensure that all of those trains are thoroughly inspected and cleared and put into service when things are ready, but there may well be a knock-on effect for some of those timetables into next week.”
He said the cracks – measuring millimetres – are on the “lifting points on the underside of the carriages used for maintenance”.
Mr Nisbet said: “It’s fair to say this didn’t pose any particular danger to passengers that were travelling on those trains but if you don’t treat these kind of issues early on then they have the potential to develop.”
Hitachi Rail said: “Safety is our number one priority and as a precaution, the decision was taken to halt the entry into service of our intercity fleets pending inspection.
“Having been cleared for service, some trains are now running again across the network.
“We are working as quickly and safely as possible to investigate the issue across the remainder of the fleets.”
Rail minister Chris Heaton-Harris said he had asked train operators to deploy extra staff to help passengers complete their journeys and access refunds.
He added: “Whilst some trains are starting to be reintroduced, disruption is likely for a prolonged period, particularly on GWR.
“I share the frustration of passengers who are experiencing significant disruption, and would ask people whose journeys are affected to check before travelling.”