Rail services around Britain are being disrupted for a third day after a fault was detected in the trains used on some routes.
Services operated by Great Western Railway (GWR) and London North Eastern Railway (LNER) have been affected since Saturday after cracks were found in the chassis of some trains.
Several other networks, including Hull Trains and TransPennine Express have also been affected after the Hitachi 800 trains were withdrawn from service for safety checks.
GWR has advised passengers with long-distance tickets not to travel on Monday and LNER has urged travellers to check before they travel as a number of services have been cancelled.
Both networks have warned that the disruption is likely to carry on further into the week.
GWR said there was "no service or an extremely limited service" between London Paddington and Bristol Temple Meads, Swansea, Penzance, Hereford and Cheltenham Spa.
LNER has cancelled services on the East Coast Main Line, affecting trains running between London and Aberdeen, Edinburgh, Leeds, Inverness and Sunderland.
Robert Nisbet director of nations and regions at industry body the Rail Delivery Group, told the BBC that the disruption was “disappointing”, particularly as more passengers were returning to the network following the easing of Covid restrictions.
Asked if he knew how long the disruption might last, he told d BBC Radio 4's Today programme: "No, I can't put an exact time on it and that is purely because we are going through the process and taking it extremely seriously.
"These trains are relatively new in service, so this is something that the Hitachi engineers will be looking at and reporting back to the train operating companies."
He said that although the tiny cracks didn’t pose “any particular danger” to passengers they could develop into a bigger problem if left untreated. He added that if cracks were found, replacement trains may have to be deployed.
Hitachi apologised to passengers affected by the disruption, saying: "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."