Fine Gael said last night it is considering lodging a complaint against the Taoiseach with the Standards in Public Office Commission (Sipo) in relation to his tax affairs.
Ethics legislation requires that TDs and senators lodge tax-clearance certificates with Sipo within nine months of a general election.
Fine Gael is querying whether, in the light of the alleged "dig-outs" of the early 1990s being examined by the Mahon tribunal, the certificate lodged by Mr Ahern at the time of the 2002 election was "improperly obtained".
Senator Eugene Regan, the party's justice spokesman in the Seanad, said the matter of a complaint to Sipo "is under active consideration" by Fine Gael.
In response, a spokesman for Bertie Ahern said: "The Taoiseach's dealings with Sipo and the Revenue are of a confidential nature and will remain so. He will co-operate with any query from any regulatory authority, including Sipo, as he did in the past."