Siptu has said that the decision by Transdev to suspend sick pay for Luas drivers is "outrageous".
The tram operator announced that all drivers - excluding those who are critically ill or can prove they are "genuinely sick" - will not be paid sick leave for the foreseeable future.
In a letter to Siptu, Transdev director Gerry Madden said absentee rates of drivers has increased almost three-fold from 4.5pc last year to 12pc this year.
He said that this is "imposing significant additional costs on the company and is highly disruptive to our customers".
As a result, the company has invoked its right to "suspend individuals from the regular Sick Pay Scheme" under an agreement with the union which allows the company to forfeit sick pay for those employees whose absentee rate exceeds 4pc, he said.
A company spokesperson said "there's no doubt about it that sick leave has increased dramatically since the start of the industrial action".
They added that all Luas drivers aside from those who can prove a medical issue will not get sick pay until "absence rates return to normal", according to the figures provided in a letter issued last Friday.
Siptu organiser Willie Noone said the company's decision to invoke the right to suspend sick pay has not only escalated the stand-off between the two sides in the long-running dispute, but claimed it could endanger lives if drivers are forced to work while sick.
"It's not surprising these people are going off sick due to stress," he told the Herald.
"If an incident occurs because a driver is fatigued, it's on their (Transdev's) heads."
He also claimed the company is targeting the entire driver grade when the clause in the agreement was intended for any individual drivers whose absentee rate exceeds 4pc in a year.
"If an individual is over- utilising it, we don't have a problem. But what they're doing is taking a whole category out and that's not in the spirit of the agreement," he added.
Mr Noone criticised the company's assertion that it will continue to judge each sickness claim on a case-by-case basis which he said is entirely arbitrary. Siptu is now looking at the matter as a health and safety issue.