Docklands Light Railway (DLR) workers are to be balloted about industrial action in a dispute over jobs and pay.
More than 250 members of the RMT union are being asked if they want to strike.
The union has accused operating company Docklands Serco of planning to cut station staff and reduce station assistants’ pay by £5,000.
Serco has denied the claims and said it plans changes that are part of a reorganisation which will increase numbers of staff at stations.
The RMT also claims the company wants to sack or downgrade more than half of the current station supervisors.
In a separate dispute about safety, the RMT is threatening to ballot London Underground staff.