Micheál Martin has written to the Taoiseach Leo Varadkar seeking agreement on a date for the next general election.
In the letter, the Fianna Fáil leader has called on Mr Varadkar to "step in and give some certainty".
Mr Martin wrote: "The responsible thing to do at this point is to end the speculation and agree a date for the dissolution of the Dail, a date for the holding of the election.
"Nothing positive can be achieved if we allow growing escalation of speculation" or "short-term tactics" concerning the calling of the election.
He added in the letter seen by the Sunday Independent that the increased speculation "ignores the need for stability and the reality of the completion of Brexit and what needs to be done over the coming months".
Yesterday, the Taoiseach announced he plans to meet Mr Martin when the Dáil term ends.
Government sources said Mr Varadkar had not read Mr Martin's letter yet as he has spent the last few days at a European Summit in Brussels.