Libertas has claimed that the Yes campaign's support for a European Parliament report on tax harmonisation shows Fine Gael and Labour cannot be trusted on the issue.
The 2005 Bersani Report called for a common tax system and was backed by MEPs from both parties.
But the former Labour finance minister, Ruairi Quinn, insisted that all the main parties later committed themselves unequivocally to defending the independence of our tax system and our low corporation tax rates.
Labour's spokesman on European Affairs Joe Costello called the Libertas claims groundless and deliberately misleading.
Libertas are headed by businessman Declan Ganley and are campaigning for a No vote.