Rail disruption is set to continue on Sunday – and possibly into Monday – 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.
There will be "similar disruption" on Sunday, GWR told Sky News on Saturday evening, adding that it was unsure about Monday.
It is in regular contact with Hitachi and is expecting an update, it said.
LNER has 65 affected trains. Early on Saturday evening it said more than half had been checked, and were being put back into service.
It did not say if problems requiring repairs had been found on any of the trains.
LNER said the checks would be complete either late on Saturday or on Sunday – and travel disruption on Sunday is "possible".
Hull Trains said normal services had resumed by 1.30pm on Saturday.
Rail minister Chris Heaton-Harris said routine checks on Saturday morning had identified "cracks on part of the chassis of some trains".
He said the trains had been taken off the network "to undergo full and rigorous checks".
"Trains will be returned to service as quickly as possible once they are fully approved as safe by the manufacturer," Mr Heaton-Harris said.
He warned that "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," the minister added.
A GWR spokesperson told Sky News there have been 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 are now being inspected as a precaution.
GWR said "there will be significant disruption across the network today" and services running to and from London’s Paddington Station have been cancelled.
There is currently 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.
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".