Rail disruption is expected to continue for a second day after services were cancelled across the country due to cracks in some trains.
Hitachi 800 trains were pulled from lines on Saturday as a “precautionary measure” after the fault was found in some trains.
Great Western Railway (GWR) and London North Eastern Railway (LNER) have advised people not to travel on Sunday.
Trains serving Edinburgh, Aberdeen, Inverness and Glasgow have been affected by the issue.
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.
A spokesperson said: “We have had to cancel a significant number of long-distance train services on Saturday and disruption is expected to continue on Sunday and into the following week.
“Train timetables are presently being worked on and will be published as soon as possible.”
LNER also urged passengers not to travel on Sunday.
A spokesperson said: “Our sincere apologies to customers affected by disruption. Please do not travel this weekend.
“Tickets dated for travel May 8-9 will be valid up to and including Sunday 16 May.”
Hitachi Rail has apologised for the disruption caused by “cracks on the lifting points under the carriage of some class 800 trains” which were spotted during routine checks, adding that by Saturday evening “some trains” had been cleared to run as normal.
A spokesperson 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.
“We understand the frustration caused and we would like to apologise for the inconvenience caused to passengers and operators.
“Having been cleared for service, some trains are now running again across the network.”
Rail minister Chris Heaton-Harris has asked operators to deploy extra staff to help passengers complete their journeys and access refunds.
He added: “I share the frustration of passengers who are experiencing significant disruption, and would ask people whose journeys are affected to check before travelling.”
The non-ministerial Government department responsible for regulating Britain’s railways, the Office of Rail and Road (ORR), said it has begun an investigation into the issue to “establish the full facts of the issue and any lessons which need to be learned”.
Mick Lynch, general secretary of the Rail, Maritime and Transport union, said Hitachi must ensure “the highest safety standards” and “properly investigate and rectify the issues”.
Manuel Cortes, general secretary of the Transport Salaried Staffs Association, said passengers should not be charged extra in future to pay for repairs.
He said: “It’s important to point out that the affected trains are relatively new, in which case the manufacturers should foot the bill for any repairs, not passengers or taxpayers.”
Anthony Smith, chief executive of independent watchdog Transport Focus, added: “Safety must always come first. However, it is very disappointing this is happening on a relatively new set of trains…
“Refunds should be quick and compensation generous. Many thousands will have their travel plans upset this weekend.”