London Underground station staff are to be balloted for strike action over plans to cut 750 jobs and close ticket offices.
The Transport Salaried Staffs' Association is balloting 1,300 members over industrial action, which could be taken from 3 February.
Transport for London (TfL) wants to have staff in ticket halls and on platforms rather than in offices.
Members of the Rail Maritime and Transport Union have already been balloted over possible industrial action. The result will be announced on Friday.
London Underground has said it would seek to avoid compulsory redundancies.
It said sales at ticket offices only accounted for about 3% of all journeys, after announcing that staff would be switched to station concourses.