A 48-hour strike on London's Docklands Light Railway will go ahead from 4am on Wednesday after talks failed to avert the action.
Transport for London has warned it is unlikely that any DLR services will run during the walkout by members of the Rail, Maritime and Transport union.
A second phase of action will coincide with the London Marathon next month.
Two days of talks at the conciliation service Acas between the union and KeolisAmey Docklands Ltd, which operates the DLR, failed to resolve a dispute over claims of a "comprehensive breakdown" in industrial relations.
RMT general secretary Mick Cash said: "RMT recognises the impact that our action will have and the responsibility for that will be entirely down to the company.
"This dispute is over the fundamental issues of workplace justice, fairness, the outsourcing of key functions and sticking to agreements and practices that Keolis seem to believe that they can trample all over.
"RMT members employed by KAD have had enough of being treated like dirt and have made it clear that they are prepared to stand up and fight for their fundamental rights."