Talks aimed at resolving a long running dispute over driver only trains on Southern Railway have "stalled", with no agreement.
Leaders of the Rail, Maritime and Transport union spent over two hours meeting with officials from Southern's owner Govia Thameslink Railway to try to break the deadlocked row.
RMT general secretary Mick Cash called for round table discussions involving all parties with a direct interest in a solution to the guards' and drivers' disputes.
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.
"RMT is proposing that we try to break the deadlock by getting both unions, the Department for Transport and the company involved in round table discussions with everything on that table.
"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."