The Green Party TD who lodged a High Court challenge against the Government says there is legal opinion that the Comprehensive Economic Trade Agreement (Ceta) is unconstitutional.
Dublin South Central representative Patrick Costello lodged the proceedings on Monday over whether a referendum would be needed on the controversial trade deal.
Climate campaigners say Ceta would hamper efforts to tackle the climate emergency because a mechanism within the deal, the Investor Court System (ICS) allows for multinational companies to sue a state for damages if it introduces new laws or policies that the company thinks will reduce its future profits.
In a statement released to thetoday, Mr Costello said: "I had sought the opinion of counsel on the ratification process, in particular the Investor Court System (ICS) element of Ceta which will come into effect if ratified.
"The ICS involves a transfer of sovereignty and of judicial power incompatible with the Constitution.
"It is the opinion of counsel that there is a good stateable case that the ratification of Ceta, and in particular the ratification of the ICS without a referendum would be contrary to Article 15 and Article 34 of the Constitution.
"It is of fundamental importance that the members of the Dáil, myself included, who would vote on whether to ratify Ceta know that our votes are constitutional."
He said he has taken the action as an individual citizen and it is a personal action.
"The treaty will be going to Oireachtas Committees for scrutiny on the question of should we ratify Ceta and what are the consequences of that.
"This legal challenge will, at the same time, answer fully the question of how we ratify this — can we do it by Dáil vote or do we need a referendum?
"Investor courts as seen in other jurisdictions are hugely problematic since they allow for the governments to be directly sued by companies outside of existing legal systems.
"This, in turn, may affect how policy makers decide policy in the long run.
"As such, it is of crucial importance that if Ceta is to be ratified, then the process used is the correct one.
"While there might be different views on this question of does Ceta need a referendum, the only place we can get a definitive answer is the courts."
The vote on the deal has already been postponed once from its initial date in 2020 when it became clear that Green Party leader Eamon Ryan would not be able to get all of his TDs to pass the bill, despite assurances to the Taoiseach Micheál Martin he could do so.
Those in favour of passing Ceta note there is a commitment in the programme for government to support trade deals, and there is an expectation that Ceta should be ratified within that remit.
It is understood that Mr Costello told his parliamentary party, including the leadership, on Monday evening. Government members were informed on Wednesday.
The Ceta deal has been a bone of contention within the Green Party for months, with TDs Neasa Hourigan and Mr Costello steadfast that they would not be voting for the deal, against which the Green Party had previously campaigned.