Southern Railway has been ordered to pay for a £13.4 million package of improvements after Transport Secretary Chris Grayling decided that industrial action does not fully explain the operator’s poor performance.

It came as the firm’s train drivers announced fresh strikes in a dispute over pay.

Members of trade union Aslef will walk out on August 1, 2 and 4 after a vote found that 62% were prepared to take part in a strike. Turnout was 81%.

Southern has been involved in a bitter row with unions over proposals for so-called driver-only operated trains, with conductors holding several strikes in the past year, while drivers have separately walked out over the issue.

But services have also been impacted by extensive railway improvement works, with Southern’s parent company Govia Thameslink Railway (GTR) admitting it had “underestimated” the impact of this.

Following the start of industrial action in April last year, GTR lodged a claim for force majeure, arguing that poor performance was due to strikes and high levels of staff sickness.

Southern Rail customers have had their travel disrupted by industrial actionSouthern Rail customers have had their travel disrupted by industrial action (Dominic Lipinski/PA)

But Mr Grayling concluded that “this does not fully explain the poor service that passengers received”.

Mr Grayling said passengers have been “badly let down” and he described the industrial action as “totally unjustified”, adding that it “must stop now”.

He went on: “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.”

Transport Secretary Chris Grayling has ordered Southern Rail to fund improvements to its servicesTransport Secretary Chris Grayling has ordered Sourthen Rail to fund improvements to its services (David Mirzoeff/PA)