Govia Thameslink Railway (GTR) is being investigated over their communication with passengers before and after the new timetables which led to major disruption.

Alongside Northern, the two train companies allegedly breached a condition of their operating licence, according to rail regulator the Office of Rail and Road (ORR).

This clause requires them to provide “appropriate, accurate and timely information” to allow passengers to plan their journeys “with a reasonable degree of assurance”.

GTR, which runs the Thameslink, Gatwick Express and Southern Railway services, is being investigated by the Government department.

Although the May 20 timetable change aimed to deliver big benefits to passengers because of major network upgrades, services in the south east of England suffered.

Both GTR and Northern cancelled up to 470 and 310 trains respectively each day during the huge disruption, which lasted several weeks.

Anthony Smith, chief executive of passenger watchdog Transport Focus, said: "Passengers were badly let down when new timetables descended into chaos on some Govia Thameslink and Northern routes.

"One thing that would have helped passengers work around these problems was accurate, timely and useful information - but in many cases that was not provided.

"It's only right that the regulator investigates if these companies breached the passenger information obligations in their licence."

The investigation is expected to be concluded by the end of next month.

An ORR spokesman said: "The opening of an investigation does not imply that we have made any findings about non-compliance by either company.

"If ORR finds the companies in breach of their licence obligations, it could result in enforcement action such as a financial penalty."

A spokesman for GTR said the company will "co-operate fully" with the inquiry.