The owner of Southern Rail has been ordered by the Government to pay £13.4m for improvements after its poor performance caused months of delays.

Transport Secretary Chris Grayling said prolonged industrial action could not “fully explain the poor service” as commuters suffered cancellations and overcrowded shorter trains on the line.

But the payout, outlined in a letter by Mr Grayling to Govia Thameslink Railway (GTR), was described as a “whitewash” and not even a “slap on the wrist” by the RMT union.

Mr Grayling said: “Passengers who depend on Southern have been badly let down.

“The union industrial action that has so often disrupted services is totally unjustified and must stop now. But GTR must also do better in providing services to its passengers.

“When trains are cancelled unnecessarily it can cause huge disruption. And when trains are shorter than they ought to be, it can leave already busy services unbearably overcrowded.”