The Green MEP, Ms Patricia McKenna, has threatened legal action against the European Commission, accusing it of "illegal and improper interference" in the Nice Treaty referendum.
A spokesman for Ireland's EU Commissioner, Mr David Byrne, accused Ms McKenna of attempting to silence free speech.
"This looks like a naked attempt to muzzle freedom of expression. Mr Byrne as an Irish citizen is perfectly entitled to express his views in relation to the Nice Treaty or indeed any other matter," the spokesman said.
A solicitor's letter sent on Ms McKenna's behalf to the Commission's Secretary-General, Mr David O'Sullivan, said the Commission was not authorised to promote the treaty in any way.
" `Promotion' includes the expenditure of European monies on promotional material . . . the diversion of salaried Commission staff from their duties to participate in events endorsing an EU treaty and debates confirming such a treaty before it is ratified and the endorsement and promotion of treaties by salaried commissioners even in circumstances where the Commissioner in question is an Irish citizen," the letter says.
Ms McKenna denied she wanted to limit freedom of expression and complained that Commission representatives in Dublin were playing an active role in the campaign for a Yes vote. She recalled that she faced similar accusations when she challenged the Government's right to use public money to promote one side in a referendum campaign.
"Paid officials from the Commission are appearing on radio programmes campaigning for the Nice Treaty. That's wrong, and I believe that Irish and EU law say it is wrong," she said.
Mr Byrne's spokesman dismissed as absurd Ms McKenna's suggestion that the Commission should refrain from speaking about any treaty before it is ratified. The Commission says it has exercised great caution during the referendum debate so far and as the guardian of the EU treaties it has an obligation to explain treaty changes.
The Commission has passed the letter to its legal advisers, but it is unlikely to heed Ms McKenna's warning until it receives firm legal advice to do so.