Glasgow Underground staff have voted in favour of a series of one-day strikes in an escalation of their pay dispute.
Staff have been pushed into strike action, the union says
The T&G union said its members would enforce a complete overtime ban from 30 January and would begin strike action a day later.
Staff held a four-hour strike on Thursday in protest over pay and holidays.
Scott Foley, of the T&G, said Strathclyde Passenger Transport (SPT) had failed to meet staff demands.
The workers have rejected a two-year pay deal worth an extra 5.9%, which SPT director Douglas Ferguson said would not be increased.
Mr Foley said underground workers operated in "a safety critical environment" without being paid for the job they carried out.
He added: "They have been pushed by the SPT management to a point where they feel they have no other option but to embark on a period of industrial action.
"Despite the repeated attempts by T&G officers and stewards to progress initiatives, including using the services of Acas, to resolve issues at SPT, management has resisted all attempts at progress, resulting in our members' anger, frustration and disappointment.
"Our members want a modernised SPT Underground, complete with a modern progressive grading structure.
"What our members need and deserve is a modern agreement, properly evaluated, that truly reflects the worth and value of the jobs they do, comparable to that of their peers in the industry and SPT must recognise this."