Unite's members look after maintenance, safety and power control
Engineers and workers on London Underground (LU) will be balloted over a pay dispute, a union has said.
Unite will hold a strike ballot after rejecting a "final" two-year pay offer from LU which it said did not reflect "real rises" in the cost of living.
LU offered workers a 1.5% pay rise in the first year followed by a 0.5% raise above the rate of inflation.
Transport for London (TfL) said it was "disappointed" but added Unite's 600 members formed 2.5% of its total staff.
The union has not announced the date for the ballot.
Regional officer John Morgan-Evans said: "London Underground presented this as a final offer, indicating to us that they are no longer interested in negotiating.
"In these circumstances we have no other option than to ballot our members on industrial action with a recommendation that they vote in favour."
Members of the union, which also includes electrical workers, look after the maintenance, safety, management and power control on the network.
The union said it is willing to return to the negotiating table.
A TfL spokesman said: "We are disappointed that the Unite leadership has chosen to ballot its 600 members at London Underground when progress is clearly being made with all other unions towards a new agreement on pay and conditions.
"We urge them to call off this ballot, which will achieve nothing."