SIPTU has warned that Dublin's Luas light rail service could face an indefinite shutdown in five to six weeks' time after talks aimed at averting further industrial action broke down.
The service was shut down by four days of strike action last month over a pay claim of up to 53%.
A further three days of strikes are scheduled for St Patrick's Day, Easter Sunday and Easter Monday.
Arriving at the Workplace Relations Commission earlier for talks, SIPTU confirmed that it had significantly reduced the scale of its pay claim.
It subsequently emerged that drivers had virtually halved their claim and were now seeking 6% per year over five years, backdated to 2014.
However, after almost 11 hours of negotiations, Managing Director of Luas operator Transdev Gerry Madden said the modified pay claim of drivers and control staff still amounted to 30% plus, which was hugely disappointing. He said no other employer was facing that kind of claim. No further talks are planned with those two grades.
However, Mr Madden said negotiations will continue with two other grades - Revenue Protection Officers and Revenue Protection Supervisors. It is understood their claims were more modest.
Asked whether there was now the prospect of a shutdown of the Luas service, he said he did not entertain that idea. He said he would engage in talks with the two groups who were anxious to get into meaningful dialogue with the company - but said what SIPTU did in response to today was entirely up to them.
SIPTU Divisional Organiser Owen Reidy said it was ironic, bizarre and reckless that the company was prepared to talk to staff when they had an ambitious and unprecedented claim totaling 53%, but was not prepared to negotiate after they had compromised by halving their claim.
He said they would continue negotiations with Transdev on behalf of the RPO and RPS grades on Friday.
However, the drivers and control staff would have to discuss their options including the possibility of a ballot for all-out strike.
He said that if an employer was not going to talk to a group of workers, and they wanted to proceed with a pay claim, they would have to take alternative action.
He said he could see a complete, indefinite shutdown of the Luas happening in the next five to six weeks' time.
He said that would be crazy and should not happen, but he said the decision the employer took today to end negotiations was extremely reckless.
Director General of the Workplace Relations Commission Kieran Mulvey acknowledged that the prospect of further disruption to services is serious.
He said he was flummoxed as to how the dispute had got to this level, but warned that it had potentially serious implications for employment.
In a statement, Transdev said: "We wish to acknowledge the assistance of the WRC in seeking to move this dispute forward."
"The revised position put forward by two groups Revenue Protection Officers and Supervisors while still significantly beyond the company position does provide a basis for further discussions."
"We are anxious to progress matters with these groups under the auspices of the WRC and further meetings will be scheduled as soon as can be arranged.
"However, the ‘bottom line’ positions put forward by the drivers and traffic supervisors do not provide any realistic basis for negotiations.
"At this stage, we are requesting SIPTU to convene a general meeting of their members as a matter of urgency to review the mandate for industrial action (including St Patrick's Day and Easter Sunday/Monday) and to establish whether the positions being put forward by the internal representatives reflects the views of the majority of their members.
"Following this meeting the company will re-engage with SIPTU to determine if a basis then exists for resolving this dispute," the statement added.