Govia Thameslink Railway (GTR) is mounting a legal challenge against a strike ballot by the drivers' union Aslef on its Southern and Gatwick services.

The company said it had issued papers at the High Court and expects the matter to be considered at a hearing next week.

The firm secured an injunction last month against Aslef in a dispute over the introduction of longer trains on the Gatwick Express.

The union subsequently pressed ahead with an industrial action ballot.

A GTR spokesman said: "We only launch legal action reluctantly, but it is our responsibility to do everything we can to seek to protect our passengers against further industrial action."

The dispute is separate to the row over the role of conductors on Southern which led to a strike yesterday.

GTR said the main grounds for the injunction are that it believes Aslef has breached rules on balloting.

The company accused the union of "inducing" drivers to refuse to drive trains before the ballot.

Since last month's injunction, GTR has been introducing 12-carriage trains on the Gatwick Express and intends to run half of weekday services with the longer cars by the end of June.

A statement from Aslef reads: "Aslef is disappointed that GTR is taking legal action against our ballot of Southern and Gatwick Express drivers.

"We have yet to take any industrial action. The result of the ballot is due on Monday, May 23, following which Aslef’s executive committee will determine the union’s next steps."