Workers on the Docklands Light Railway have begun voting on whether to take industrial action in a row over redundancies, duty hours and pensions.
The Rail, Maritime and Transport union claimed Serco, which runs the service, treated workers with "contempt" and did not want to hold "proper negotiations".
The union said its 400 members will vote over the next few weeks.
Serco Docklands said the ballot was unnecessary as it was making a "genuine attempt to resolve the issues".
The RMT said the management has failed to inform them whether it would seek to make redundancies and how many jobs were under threat and discuss the new pension arrangements "imposed" on workers.
The union has also called for a 35-hour working week.
David Stretch, managing director of Serco Docklands, said: "We have been working constructively with union representatives over the last few months to address a number of issues and concerns and to establish an improved way of working together.
"Despite a genuine attempt to resolve these issues and avoid disruptive industrial action taking place, the RMT has taken the unnecessary step of calling for another ballot of their members."
Further talks were scheduled with the union, the firm said.
RMT general secretary Bob Crow said: "Management at Serco Docklands continue to show total disregard for the idea of proper negotiation and consultation with this trade union on a number of issues.
"A strong yes vote in this ballot will show management we mean business and that we will not accept their disgraceful attitude towards their staff."
The results of the ballot will be announced on 11 January, the union said.