The RMT said Southern rail strikes will go ahead next week, after the union said it was "excluded" from talks between Aslef and rail bosses.
Southern drivers and guards who are members of the RMT union will strike on Monday 23 January.
RMT general secretary Mick Cash said: "RMT demands again today that this union be given access to exactly the same talks process in our Southern rail disputes as has been brokered for our sister union by the TUC."
Read more: £49k a year, four days a week: Southern rail wants you
"We would remind all parties that not only is RMT a recognised drivers' union on Southern but that it is also our guards members who have made huge personal sacrifices in the long fight for passenger safety," he added. "The notion that some sort of deal can be done which leaves those Southern guards out in the cold is ludicrous."
Train drivers' union Aslef had suspended industrial action planned for next week after Southern agreed to talks, earlier this week. Strikes had been planned for the 24, 25 and 27 January.
The union's general secretary Mick Whelan said: "Aslef believes that the best way to resolve the dispute at Southern is for all parties to engage in meaningful talks - without preconditions - aimed at finding an agreed way forward."
The RMT said it had written to the Trades Union Congress (TUC) - whose leader, Frances O'Grady, chaired the talks - in hopes of being involved in the discussions. If it wasn't included in the talks, the RMT said, its strike planned for next Monday would go ahead.
A TUC spokesperson said: “Aslef asked us to intervene in its dispute with Southern Rail, which is separate to the RMT’s dispute with the company. Right now we are focused on trying to resolve Aslef's dispute.
“We are of course keen to assist RMT in any way we can and will maintain close contact with them.”
Southern rail users have been subjected to a series of strikes over the last six months as a row over plans to remove conductors from trains rages on.
The rail company is trying to solve its service disruption problems with a hiring spree - its owner Govia Thameslink Railway launched a new advertising campaign for full and part-time roles earlier this week.
When talks were held in December, RMT general secretary Mick Cash was angry over talks going on without his union's presence.
Southern said Aslef, representing nearly 1,000 drivers, had been invited by conciliation service Acas for talks, while the RMT, representing 12 drivers, weren't involved in the discussions.