Southern Railway is taking legal action to try to stop a series of strikes from Tuesday, according to the union at the centre of the bitter dispute over the role of conductors.
The Rail, Maritime and Transport union said it had received a legal challenge, just hours before the start of a three-day walkout.
Union members on Southern are due to strike from one minute past midnight tomorrow, causing fresh travel misery for passengers.
RMT general secretary Mick Cash said: "RMT can confirm the receipt of a formal legal challenge from Southern Rail in the guards safety dispute.
"It is appalling that rather than sitting down with us at Acas today to seek a resolution the company have chosen to run to the courts under the cloak of the anti-union laws.
"The union intends to continue with the planned action and is examining the details of the paperwork . We will issue a further statement in due course."
A Southern spokesman said: "On Friday they told their conductor members to accept a deal, and then tomorrow they plan to strike against it.
"It is a situation which will leave our passengers baffled, and in that context we are presently considering all possible options to stop the strike."
Southern added: "We have written to the RMT about the validity of the dispute in light of recent developments. We await their response and will consider all possible options to stop the strike and that includes a legal challenge."