Control room staff on London Underground are to be balloted for strikes in a row over pay.
The Transport Salaried Staffs Association said it was issuing ballot papers to its members in a dispute over maintenance responsibilities being "loaded" on to controllers.
Talks are to be held at the conciliation service Acas.
TSSA general secretary Manuel Cortes said: "Disappointingly but predictably LU are continuing to drag out this unnecessary dispute and, whilst we will enter the Acas talks in good faith, we are getting on with sending out the ballot papers and will take industrial action if LU force us into it.
"LU are still guilty of breaking all agreed procedures with the control service and our staff have run out of patience and are prepared to take action because LU have spent four months refusing to negotiate."
The warning came after members of the Rail, Maritime and Transport union voted to accept a deal for an all-night Tube service.
Press Association
