An Post's decision to sell Fianna Fáil the rights to have its name and message on all stamped letters over the next 10 weeks could be open to a legal challenge, according to the chairman of the Chartered Institute of Arbitrators.
"I think there is a prima facie case to to be made that they are tampering with the mail," said Mr Roy Sherlock.
"For An Post to put advertising on people's letters and particularly political advertisements like that is a totally inappropriate and outrageous thing to do."
But a spokesman for An Post rejected claims that it was tampering with the mail by offering advertising through its stamp cancellation procedure.
"We're not tampering with the mail. We're simply protecting our revenues by cancelling the stamps and using a facility that new technology gives us. Postmark advertising is used all over the world, not just in Ireland.
"We are a commercial company and we have made a commercial decision that we're going to use it in this particular way and we're satisfied this is the way to go," he said.
However, Mr Sherlock said An Post's move was damaging to organisations such as his own which relied on their reputations of impartiality.
He said the institute had received calls from people who genuinely thought it was promoting Fianna Fáil.
"Arbitrators not only have to be impartial but have to be seen to be impartial and we cannot have the perception that we're promoting anything, let alone a political party," he said.
Extending the advertising to the political arena was "entering dangerous ground and was extremely inappropriate," said Mr Sherlock.
"We are the Irish branch of the Chartered Institute but we encompass the whole island.
"Over 100 of our members are in the North. That means hundreds of letters we send out to people in the North are promoting Fianna Fáil, the Republican Party, and that is totally unacceptable to us," he said.
A caller to RTÉ's Liveline programme said his small manufacturing business, which exports to Northern Ireland and which advertises through mailshots, had seen its business there drop by 50 per cent because of the advertising.
Other political parties have reacted angrily to the deal between An Post and Fianna Fáil. Labour's spokesman on Public Enterprise, Mr Emmet Stagg, said the arrangement should be ended. "Whatever argument there may be for An Post using space on envelopes to advertise charities or other non-controversial bodies, there is no case for using post to promote a particular party," he said.
Fine Gael Public Enterprise spokesman Mr Jim Higgins TD described it as an abuse of private property. "A letter is the personal private property of the individual who sends it, and for An Post to allow a political party print its propaganda on the envelope is an interference with that peron's right of free association and party allegiance of their own choice."