A Labour senator is threatening to take An Post to court unless it stops stamping his post with the Fianna Fail logo.
Senator Joe Costello has instructed his legal team to serve a plenary summons against An Post and is giving them seven days to respond.
He said if An Post fails to end the contract with Fianna Fail, he will seek a court injunction next week.
"If they are engaged in an illegal act, which I believe they are, at this point every letter I send to my constituents, the first thing they see is a message from an opposing political party," he said.
"That's unfair, undemocratic and that's wrong".