Luas drivers have been put on protective notice in the on going row over pay.
Siptu says it marks an escalation in the dispute and makes the possibility of reaching a negotiated settlement more remote.
An all-out strike now appears to be on the cards after Transdev and Siptu failed to break the deadlock at last-ditch talks yesterday.
Transdev wrote to employees today after talks failed to end the deadlock yesterday.
The company has warned of pay cuts and job losses as a result of the continued strike action.
In a letter to employees, Transdev warns staff will be employed on a day-to-day basis and that an "appropriate financial deduction" will be imposed for "unacceptable part-performance of duties".
The letter warns that any worker who does not carry out their full duties will be deemed to have removed themselves from the payroll.
Transdev also said that the cost of any industrial action after April 24 will be offset by pay cuts.
The company have issued an April 17 deadline to workers to accept a pay offer which is lower than the offer proposed in the Workplace Relations Commission deal that was already rejected by staff.
Any future offer will be even lower.