Micheál Martin has said he "wouldn't trust" Leo Varadkar not to go into government with Sinn Féin.
The Fianna Fáil leader said "Leo will do whatever it takes to get himself back in as Taoiseach".
Mr Martin said there are a number of outgoing Fine Gael TDs who have said they are willing to consider a coalition with Sinn Féin.
The Fianna Fáil leader also claimed Mr Varadkar timed the election to coincide with Brexit day, in the hope his G=government would be "re-elected on the wave of Brexit day".
Speaking to RTÉ News in Rathfarnham, Mr Martin said;
"He has planned this election and the day, away back. It's very clear now this was all choreographed. He thought he would be re-elected on the wave of Brexit day".
He said Fine Gael has been "desperate" since the campaign begun to push Brexit centre stage.
Mr Martin was responding directly to a comment from Mr Varadkar that Brexit was being raised by voters on the doorstep.
He said that was not the case, claiming people are more concerned about health and housing. He said teachers and students are concerned about education.
Mr Martin said the Fine Gael effort to push Brexit as an issue "hadn't worked".
