Southern rail will meet the RMT for fresh face-to-face talks only if the union suspends its planned industrial action on 4 April, the company announced on Monday (20 March).
Southern operator Govia Thameslink Railway (GTR) has been in dispute with the RMT for a year over the future role of rail guards on the franchise.
GTR, in a modernisation move, wants to introduce driver-only-operations (DOO) across the network, with the RMT raising job warnings and safety concerns.
"Our passengers, staff and the regional economy want certainty and an end to your industrial action, not a 24-hour truce," said Charles Horton, the chief executive of GTR.
"It's disappointing that the RMT are now threatening a 31st day of strike action on 4 April. Over the past year we have met with the RMT countless times to try and settle your dispute. Unfortunately, each time has ended in disappointment."
The development comes after GTR and drivers' union Aslef agreed to a new peace deal. The union's members on Southern rejected the last draft agreement between the parties in a referendum.
The result of the latest referendum will be announced on Monday 3 April. Details of the agreement have not been made public, but RMT chief Mick Cash described it as "basically the old deal in a new envelope".
"We have now had a chance to look at the detail and this is basically the old deal in a new envelope and RMT does not believe for a moment that drivers and guards will be hoodwinked," the general secretary said.
"Drivers and passengers have still lost the cast-iron guarantee they once had of a guard on the train which means they will now be more exposed and left vulnerable when something goes wrong. Disabled passengers will be seriously disadvantaged as access to train services is compromised."