The owner of Southern rail says it will take the union Aslef to the Supreme Court over its industrial action on the train network.
Drivers are continuing a 48-hour strike, with another walkout planned for Friday. Almost all of its 2,200 daily services have been cancelled.
Further strikes by Aslef have been called for 24, 25 and 27 January.
Southern’s owner Govia Thameslink Railway (GTR) lost a court case and an appeal last year over halting strikes.
Southern has insisted it will not back down in the dispute over who should open and close train doors.
‘Door always open’
In a statement GTR said it was “determined to protect its passengers and its business from unlawful industrial action”.
It added: “GTR is therefore prepared to continue its legal claim to the Supreme Court, as it believes that it has an arguable case that the industrial action is unlawful under EU law.”
Mick Whelan, the general secretary of Aslef, earlier said it had not had any conversations with Southern this week, but its “door was always open”.
“I think it’s the policy of both the DfT and the company that they won’t talk to us in a week of action,” he added.
“If they had come to us for talks, or been willing to talk to us, we would have spoken to them.
“We’ve never ever refused a meeting and will not be doing so in the near or anytime in the future.”