Fianna Fáil has thrown the Government's plans to alter tax bands in next month's budget into doubt.
Leader Micheál Martin says the confidence and supply agreement underpinning the Government only allows for reductions in USC.
Fianna Fáil finance spokesman Michael McGrath denied the party was bluffing.
“If the agreement is breached, and it’s a very clear breach of the agreement, the obviously that threatens the operation of the whole agreement and would threaten the scenario whereby a Budget is going to be passed by way of us abstaining,” he said.
“And that issue would come down to a discussion ultimately between the Taoiseach and our party leader Micheál Martin – but we’re looking to avoid a situation where anything gets to that point.”