A train company has been threatened with legal action after comparing its service to Poundland chocolate.
Thameslink said it was "very sorry" for making the comment in response to a disgruntled passenger.
The operator has suffered major disruption following the introduction of a new timetable on May 20.
Thameslink's parent company Govia Thameslink Railway (GTR) saw more than 450 of its trains either cancelled or at least half an hour late on Wednesday, representing 13% of total services.
On some of its routes as many as half of all trains were cancelled.
A passenger named Kevin tweeted an image of a departures board showing cancellations trains with a sarcastic caption which read: "Why, Ambassador @TLRailUK, with this fine service you are really spoiling us."
Thameslink replied: "Very sorry Kevin. Appreciate at the moment the service is less Ferrero Rocher and more Poundland cooking chocolate."
The budget store's retail director Austin Cooke issued a scathing response to GTR chief executive Charles Horton.
Mr Cooke wrote that GTR has "no right to use our name to describe poor service", stating that Poundland served eight million shoppers last week and has a "pretty good idea about what great customer service is".
He added: "If you don't want to hear from our extremely twitchy legal team, we suggest you remove your tweet."
Thameslink deleted its original message and sent another tweet which read: "Very sorry team for using your name here. I have removed the offending tweet."
The rail timetable is updated twice a year, but the latest version has seven times more changes than normal due to extra services and capacity, and a bid to improve punctuality.
GTR and Northern services have been plagued with disruption since it was introduced.
Transport Secretary Chris Grayling claimed the rail industry had "failed the passengers it serves".
In a letter to MPs, he wrote that the new timetable was finalised "much too late to permit adequate logistical planning" due to delayed Network Rail infrastructure works.
Mr Grayling added that the way Network Rail creates timetables is "simply unacceptable".
Both Network Rail and GTR apologised for the problems caused.
Mark Carne, Network Rail's chief executive, said: "There is no doubt that the May timetable was finalised significantly later than normal for reasons that were both within and without our control."
Mr Horton added: "We always said that delivering the biggest timetable change in generations would be challenging - but we are sorry that we have not been able to deliver the service that passengers expect."
He said the industry is "working very hard to bring greater consistency to the timetable as soon as possible".