Talks aimed at resolving a long-running dispute over driver-only operated trains on Southern rail have "stalled" with no agreement, the RMT union said.
It follows more than two hours of talks between leaders of the union and officials from Southern's parent firm, Govia Thameslink Railway (GTR).
RMT general secretary Mick Cash called for round-table discussions involving all parties to now take place.
"This dispute can be resolved, we have no doubt about that," he said.
"RMT entered the talks in good faith today following our discussions with Transport Secretary Chris Grayling.
"However, it is clear that there is a major blockage to making progress and that is the failure to get all parties around the table at the same time," Mr Cash said.
Both the RMT and Aslef unions have been embroiled in a dispute with GTR over the decision to replace conductors with "on-board supervisors".
Since January a number of routes have been operating this way, with extra responsibilities - including the closing of train doors - switching to drivers.
Mr Cash said the only way to try to break the deadlock would be to get both unions in discussion together with the Department for Transport and GTR.
"Meeting in different rooms at different times simply isn't working.
"RMT will be taking this proposal back to Chris Grayling and his officials with a request that they, as the contract holders, take action to broker those round table talks at the earliest possible opportunity.
"This dispute can be resolved, we have no doubt about that, and the fresh approach we have outlined today will allow us to kick start the talks process."