Talks to end strike action by Metro drivers in a dispute over pay and conditions are at an "impasse".
Drivers walked out in December for two days over proposed contract changes.
But further action was halted after the RMT and Aslef unions agreed to Metro operator Nexus' offer of an 18.5% pay rise for more flexible shifts.
However, the RMT said the deal over conditions had fallen through and drivers "may" consider further strike action.
Aslef, the second union representing Metro drivers, said negotiations with Nexus will "have to start all over again".
The disagreement concerns drivers' flexible spare days off, when they can be called in if another driver is absent on short notice.
Nexus wants to give eight days notice to drivers considered to be "spare" on the roster, but unions said this would make it difficult for workers to plan their lives especially childcare.
The RMT wants drivers to get a 28-day notification period - but that has been rejected by Metro bosses.
RMT regional organiser Micky Thompson said: "It is fair to say that we are at an impasse and with no deal we may have to consider action.
"We have worked tirelessly since December and were 99.9% there, but this one issue is preventing us from signing off this agreement."
He added that drivers were "constantly" called in on days off.
A spokesman for Nexus said: "We do not feel that we are very far apart on reaching an agreement.
"These discussions are ongoing and we are working hard towards an amicable solution to the final remaining issue on train crew rostering."