SIPTU general president Jack O'Connor has threatened "war" if no suitable agreement is reached in the Croke Park talks.
Mr O'Connor said all sides would have to come to a negotiated settlement, or the Government would face protracted public service strike action.
He said: "If we find ourselves in a space where people on the other side expect to achieve more than the absolute minimum that is required, they will stumble into a situation in which we will have to go to war, and it won't be pleasant," he said.
The talks between government representatives and public sector unions could go on past tonight's deadline set down by the Labour Relations Commission.
Today's talks are centred on increasing the working week to 37 hours from 35 hours and placing a three-year freeze on incremental pay increases for public service staff.
Minister for Transport Leo Varadkar has said he is confident agreement could be reached on proposed changes to working hours and pay cuts for public sector workers, but that it may not happen tonight.
Minister Varadkar said there was scope to push the deadline to the talks out further, if required.
"I don't know if it can be sorted out tonight…(but) decisions just have to be made…Whether you make them this weekend or in three days' time doesn't make any difference," he said.