Drivers who operate new trains on three Tube lines are to be balloted in a row over working conditions, a union said.
The Rail, Maritime and Transport Union will ballot S Stock drivers on the Circle, Metropolitan and Hammersmith and City lines over industrial action.
The union has said drivers do not get enough time to prepare trains and it is concerned about training of drivers.
Transport for London said talks were ongoing and they were surprised about RMT's decision to ballot.
The union said it was concerned about the length of time drivers get to train on the new fleet, drivers working on other Tube lines and London Underground's decision to remove train maintenance depots.
The RMT has said it will soon ballot its members working at Barking, Edgware Road, Hammersmith, Harrow, Neasden and Rickmansworth for action with the possibility of a strike.
The union's general secretary Bob Crow said: "RMT will not allow the introduction of the new S Stock to be used as a cover for bulldozing through fundamental changes to working conditions and practices that we believe will seriously impact on passenger and staff safety.
"We are appalled at the gung-ho attitude of LU to our range of carefully laid out concerns and it is that attitude that has forced us into dispute."
Nigel Holness, London Underground's director for operational development and upgrades, said: "We have been working closely with the RMT to resolve issues regarding the introduction of the new trains that are now running on the Metropolitan line and currently being introduced on the Hammersmith and City line.
"Discussions are still ongoing, however we are surprised that the RMT leadership have decided to ballot their members considering that they have not raised all the issues with us through the agreed channels."