SIPTU has said a shutdown of the Luas service seems "inevitable" following the company's decision to place staff on protective notice in the wake of ongoing industrial action.
SIPTU divisional organiser Owen Reidy said Luas operator Transdev will cripple the service by its own actions.
The company has placed all staff on protective notice with immediate effect and has warned of pay deductions to offset the cost of their industrial action.
Speaking on RTÉ's Morning Ireland, Mr Reidy said it is not "reasonable, sensible or logical" to issue an inferior set of proposals to staff following their rejection of a better deal.
He described it as a "provocative act".
Luas employees have staged strikes on eight days since February and action on a further eight days is planned for the coming weeks.
Full Luas shutdown 'inevitable' after company’s 'provocative act' towards staff: SIPTU https://t.co/LuVsx5jlbQhttps://t.co/Xg1PIffu2X
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Mr Reidy said that the dispute has entered a "new area".
He added that employees will make a decision on balloting for all-out strike action over the next couple of days.
"What we're not going to do is fall into a trap that Transdev would like to happen where we have some knee-jerk reaction.
"We'll consider this carefully but it's something that's seriously on the agenda."
Mr Reidy again criticised Minister for Transport Paschal Donohoe and Transport Infrastructure Ireland for not playing a greater role in the dispute.
