Union bosses have warned of a possible "all-out indefinite strike" on the Luas which carries up to 90,000 passengers across Dublin every day.
Frosty relations between workers and management at the capital's tram service have got even colder after the latest attempts to resolve a pay row broke down.
Operator Transdev met trade union Siptu at the Luas Red Cow depot on Monday afternoon.
But in a statement after the meeting, the company said there was no point in further discussions.
"Based on the meeting this afternoon it is apparent that there is no basis for any talks or other intervention in this dispute at this point," a Transdev spokeswoman said.
Owen Reidy, of Siptu, said the latest collapse in talks has left the two sides further apart than ever.
He said Transdev was threatening to take legal action against drivers and inspectors for allegedly breaching their contracts and was poised to notify them of their plans in a letter.
Such a move could lead to lay-offs or workers being put on fewer hours, it is feared.
Transdev is also threatening to take money from workers' salaries to make up losses caused by the action, the union claims.
"This is unprecedented, an escalation and unacceptable and could lead to a lockout," said Mr Reidy.
"We have asked Transdev not to issue the letter, it would be a real escalation and it will prolong the dispute, not resolve it."
Mr Reidy said if the company threatens such action there may be a ballot among workers for a full strike.
"If this comes to pass it has very serious consequences," he said.
"This is not just some private company, it has a very significant state contract involving taxpayers' money and this should not be tolerated.
"It could lead to an all-out indefinite strike."
Transdev said it took the opportunity during the latest face-to-face meetings with union bosses "to outline the serious consequences of this dispute continuing".
It said it was writing to Siptu and workers to set out its position.
"The union's expectations remain in excess of what was proposed at the Workplace Relations Commission (which in any event is withdrawn) and on that basis any discussions would have no prospect of success," it added.
The company said it would make no further comment until it "formally confirms our position" to the union and staff.
Talks between the sides at the WRC broke down before Easter.
The deepening row centres on drivers and inspectors asking for pay rises ranging from less than 10% to 52% for those on lower scales.
Transdev claimed a deal put on the table at the WRC would have given "substantial increases", including a driver on 42,247 euro earning 50,000 euro by January 2019, alongside a bonus of 3,250 euro.
With no resolution in sight, Siptu last week announced further strike action to the eight stoppages that have already been staged.
Five 24-hour strikes are planned on the tram lines for Thursday April 28, Wednesday May 4, Friday May 13, May 20 and May 27.
A separate four-hour stoppage is also planned for Thursday May 26.