The Government has been accused of giving "favourable treatment" to the owners of Southern Railway after deciding not to publish the results of an assessment of the company's claim of exceptional circumstances for disruption to services.

The company claimed so-called ‘force majeure’ – unforeseen circumstances preventing it from fulfilling a contract – for the ongoing problems to its services.

Services have been hit for months by industrial action, a shortage of staff, high levels of staff sickness and other issues.

Rail minister Paul Maynard, who promised to update the transport select committee on the claim, said the Department for Transport's (DfT) initial assessment has now been completed.

In a letter to committee chairwoman Louise Ellman, the minister said Greater Thameslink Railway (GTR) must be given the chance to challenge the assessment.

He wrote: "Owing to the unique circumstances on this franchise, there is no clear precedent about how to assess the situation, and it is understandable and correct that GTR have the opportunity to do this.

"I am, therefore, not proposing to publish the results of the claim at this stage until I am clear that a) GTR are not challenging the outcome and b) if they do challenge the outcome, discussions have completed.

"In the event of challenge, it would not be appropriate to publish the outcome of our analysis of the first three periods of the Force Majeure claim at this stage."

Mick Cash, general secretary of the Rail, Maritime and Transport union (RMT), said: "The Department for Transport has assessed the first three reporting periods of the GTR claim but won't publish the findings - that is simply outrageous.

"Apparently, they need to give GTR the chance to challenge the findings.

“That is yet more favourable treatment for GTR and a totally opaque process that indulges the Government's anti-trade union urges, at the expense of staff, safety, passengers and the taxpayer."