Greater Anglia has responded to criticism by the Office of Rail and Road (ORR) over the use of office workers who were trained up to work as guards during the recent industrial action by members of the RMT.
The Chief Inspector of Railways has written to the rail operator having identified a "number of areas with deficiencies" that need to be addressed.
Train Service Director for Greater Anglia, Richard Dean, said he was somewhat disappointed over the letter: "We met the ORR and we took them through our arrangements.
"We believe 100% that our arrangements were robust and we are following that up with the ORR.
"We said to them we would not use our contingency conductors until such time as we'd satisfied them that the arrangements were robust.
"We made that offer to them, they didn't tell us to do it.
"What we are actually talking about is a piece of administration that they want to see ie they want us to write up what we did, and that's exactly what we have done.
"We'll be sitting down with them and we'll be going through in as much detail as they want until they are satisfied.
"I am absolutely satisfied that our arrangements were 100% robust."