Rail disruption is set to continue beyond the weekend after cracks were found on high-speed trains.
Great Western Railway (GWR) said dozens of Hitachi 800 series trains across several operators had been taken out of service "for checks as a precautionary measure".
London North Eastern Railway (LNER), Hull Trains and TransPennine Express (TPE) have also been affected.
Rail minister Chris Heaton-Harris said routine checks on Saturday morning had identified "cracks on part of the chassis of some trains".
A GWR spokesperson told Sky News there were reports of "hairline cracks on the suspension" of some trains but insisted the issue "doesn't affect the structural integrity of the carriage".
All 93 Hitachi 800s in GWR's fleet were being inspected as a precaution and disruption on services was expected to continue on Sunday "and into the following week".
The train operator advised people not to travel on Sunday and said it would be providing refunds.
"While local services are running, we expect them to be very busy. Rail replacement services are extremely limited," GWR said.
LNER also advised passengers not to travel on Sunday, and said tickets would be valid until 16 May or they can claim a refund.
The company said more than half of its 65 trains requiring checks had been completed on Saturday evening and they were being put back into service.
It did not say if cracks requiring repairs had been found on any of them.
GWR reported "significant disruption across the network" on Saturday, with services running to and from London's Paddington Station cancelled.
There was no high-speed service on routes between London and Bristol, Swansea, Penzance, Hereford and Cheltenham, it said.
The company added that there would be "only an extremely limited service with no alternative means of transport available" for those routes.
LNER told passengers on Saturday "please do not travel" as its services were delayed and cancelled.
It meant limited to no service on the East Coast, between Edinburgh, Newcastle, York and London.
Tickets for Saturday journeys will be valid up to Sunday 16 May, LNER said.
TPE said its Nova 1 trains had been affected by the issue and were being checked. It advised passengers not to travel on its services between Newcastle and Liverpool on Saturday.
Hull Trains said normal services had resumed by 1.30pm on Saturday.
Hitachi apologised to passengers affected by the disruption.
The company said: "Some Class 800s have been taken out of passenger service as a precautionary measure to allow for thorough investigations.
"We are working with partners to resolve this issue as quickly and safely as possible. We would like to offer our sincerest apologies to passengers impacted."
The UK's fleet of Hitachi 800 trains entered service in 2017, with the company saying the design was "highly appreciated for its symbolic image as the fastest high-speed train in the UK at the time".