An post is facing fresh demands to end a contract with Fianna Fáil which has seen the party's logo and electoral messages franked on millions of private letters.
Election candidate Senator Joe Costello, of the Labour Party, said yesterday he was having a plenary summons served on An Post which seeks an end to the deal within seven days.
He says he will seek a court injunction next week if his demand is not met.
The senator, who is trying to regain the Dublin Central seat he lost in the last election, argues the arrangement, for which Fianna Fáil paid about €9,500, amounts to unlawful tampering with private mail.
"Letters I send are my property until they are delivered to the recipient. When I pay for my stamp, An Post have a contract with me to deliver that letter - that's all - anything else amounts to tampering," he said.
Mr Costello also claims the contract interferes with his right to canvass and could damage his election chances.
"I sent a letter to a constituent and when I checked if he got it, he said he only got an envelope with a Fianna Fáil logo on it and he threw it in the bin."
He claimed An Post was abusing its monopoly because candidates had no option but to use its service.
The envelope advertising service can be booked by any company, organisation or individual. This is the first time a political party has used it.
Fianna Fáil booked it from the first week in March to the last week in May.
Business letters already stamped with a franking machine - about two-thirds of all letters - are not affected.
That leaves around a million letters a week delivered with the Fianna Fáil logo and either of two messages, "Peace, Prosperity, Progress" or "Serving the Future".
This is the second time An Post has been threatened with a legal challenge.
A barrister and Fine Gael candidate, Mr Colm Mac Eochaidh, threatened a similar move last month but later abandoned the attempt.
A spokeswoman for An Post said any correspondence from Senator Costello would be passed to the company's legal officers but the legalities had already been thoroughly checked out and An Post was completely within its rights.