Strikes by Southern Railway guards and drivers in the RMT will go ahead next week after its "exclusion" from talks between Aslef and the company, the union announced.
Guards in the Rail, Maritime and Transport union will walk out for 24 hours on Monday in their long-running dispute over changes to their role.
The union also has a small number of members who drive Southern trains, and they will strike on Tuesday, Wednesday and Friday next week.
The main drivers' union, Aslef, has suspended its planned strikes next week, and called off an overtime ban, while it holds talks with the train operator under the co-chairmanship of the TUC.
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.
"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.
"The notion that some sort of deal can be done which leaves those Southern guards out in the cold is ludicrous.
"As a result of our exclusion from the talks process set up by the TUC and the Government both our guards and drivers' action remains on next week and we demand and expect a positive response to our fresh demand for a seat at the negotiating table."