Almost 700 workers on London's Docklands Light Railway (DLR) are to be consulted on whether to take industrial action over changes to working practices.
The Rail, Maritime and Transport (RMT) union is expected to announce its members' decision on 22 October.
General Secretary Mick Cash has accused employers Keolis Amey of trying to "bulldoze" through new conditions.
Keolis Amey Docklands described the ballot as "unnecessary".
It has operated the line since 2014 on behalf of Transport for London, taking over the franchise from Serco.
The dispute centres on the company's use of agency staff and what the RMT claims is the "casualising" of key jobs on the line.
Mr Cash said: "Our members on DLR are furious at the way that Keolis Amey are trying to bulldoze in some of the worst working practices and conditions."
Kevin Thomas, managing director of Kelios Amey Docklands, said he was "disappointed" that the RMT plans to hold a ballot.
He added: "This action is unnecessary and we are working to ensure we continue to deliver a normal and reliable service to DLR passengers every day."
The firm is continuing to talk to the union, according to Mr Thomas.
The DLR, which connects the City and the Docklands, has used computerised trains since it opened in 1987.
Any industrial action would be likely to involve train captains, controllers and signallers on the line.
- Published26 May 2015
