Further talks to avert a strike on Nottingham's tram system over the festive period are due to take place later this week. The dispute arose after drivers and conductors were told they would have to work later than 8pm on Christmas Eve and New Year's Eve.
Discussions between tram operator NET and the GMB broke down without agreement last week.
The GMB is balloting its members on what action to take.
Late working staff would receive extra pay but the union refused the offer made to its members.
The company says NET employees would not be alone in working past 8pm on both nights.