A rail workers' union has called for Greater Anglia to be stripped of its franchise after safety "deficiencies" were uncovered over the use of replacement staff during a strike.
The Rail, Maritime and Transport union (RMT) criticised Abellio Greater Anglia, one of five train companies embroiled in disputes over the role of guards.
The union said it had received a letter from the rail safety regulator, ORR, revealing it had identified a number of areas with deficiencies which needed to be addressed including training and safety validation during a recent strike.
The letter said: "AGA have acknowledged the failings in the above areas and agreed that contingent staff will not be used until the issues have been rectified to the satisfaction of the ORR."
RMT General Secretary Mick Cash has written to the company, saying: "That you have not demonstrated your ability to carry out such basic items of health and safety management must bring into question the suitability of your organisation to run passenger services in the first place."
He added: "Even the rail safety regulator, which is funded by the train companies, has been forced to admit that Greater Anglia's strike breaking operation was riddled with safety deficiencies and failings which meant that passengers and staff who were on Greater Anglia services on strike days on October 3 and 5 were placed at a severely increased risk.
"The regulator has drawn its conclusions after prior warnings from RMT about the company playing fast and loose with safety and after the union submitted formal complaints detailing the failures during the strike action.
"RMT is now calling on the Secretary of State for Transport Chris Grayling to intervene to protect passengers and workers from Greater Anglia's appalling disregard for safety and strip the company of the franchise."
Richard Dean, Greater Anglia Director of Train Service Delivery, said: "We are working with the Office of Rail and Road to take them through the full range of contingency arrangements we put in place in order to run a full service for our customers during RMT industrial action.
"The conductors used on the RMT strike days received the same standard of safety training as our usual conductors. We informed the ORR of our contingency plans before the RMT strike days.
"ORR carried out inspections around the Greater Anglia network on the second day of RMT strike action and observed Greater Anglia operate a full service with no incidents whatsoever.
"We will never compromise the safety of our customers. During the two RMT strike days our conductors safely ran over 1,000 services."
An ORR spokesman said: "The ORR remains committed to protecting the safety of rail passengers and staff."