Senator Jim Walsh has resigned the Fianna Fáil whip over his opposition to the Children and Family Relationships Bill and the same sex marriage referendum.
He informed Fianna Fáil leader Mícheál Martin of his views earlier.
In his letter, seen by RTÉ News, he said he would be tabling amendments to the Children and Family Relationships Bill and also the Marriage Equality Bill.
He said he had decided to press ahead with the amendments as they were connected to his deeply held and conscientious opinions.
He said he had consistently articulated his views since at least 2005; in the party and also in the Seanad.
In the letter he also told Micheál Martin that "to avoid any difficulty for the party", he thought it was advisable that he "relinquish" the party whip in the Seanad.