The results of ballots on strike action involving Rail, Maritime and Transport union members working on the DLR are set to be announced on May 12.
The RMT is in two separate disputes with service operator KeolisAmey Docklands and third party Interserve who employ the staff concerned.
The disagreements involve this year's pay deal for direct employees and a row over wages for workers at facilities company Interserve.
Transport for London has urged all parties to find a resolution to the problems.
TfL director Rory O'Neill said: "This is a dispute between the RMT leadership and KeolisAmey Docklands and Interserve who employ the staff concerned.
"We urge all parties to sit down and talk through the issues to try and resolve them as soon as possible."
The RMT's general secretary, Mick Cash, said his union intended to send a "clear message to management".