A complaint against Taoisech Bertie Ahern has been formally lodged with the Standards in Public Office Commission (Sipo) by Fine Gael.
The complaint about the Taoiseach's inability to show his tax compliance was signed by Senator Eugene Regan, who has consistently attacked Mr Ahern over his financial affairs since the general election in May.
The decision to make the complaint was made at a Fine Gael front-bench meeting this week, it emerged today.
Sipo, which is the independent ethics watchdog for members of the Oireachtas, can only investigate a TD or senator on foot of a complaint.
Mr Regan said now the complaint has been lodged the Commission can assess whether Mr Ahern has upheld "high levels of ethical behaviour, legislative compliance and conduct in public life."
Under Oireachtas rules an elected representative must submit a tax clearance certificate or a statement of application acknowledging outstanding tax issues to the Sipo.
Mr Ahern told the Dáil last year that he had no tax issues emerging from payments he received in 1993 and 1994 which are the subject of his evidence before the Mahon tribunal.
But it was reported earlier this month that he would not be obtaining a clearance certificate and had lodged a €70,000 surety with the Revenue Commissioners.
Mr Regan's complaint relates to the tax clearance certificate Mr Ahern lodged with Sipo after the 2002 election.
"We now know that the Taoiseach has tax compliance issues dating back to the 1990s. This raises the serious issue of whether or not he properly complied with his obligations under the ethics legislation after the 2002 general election.
"As with every other TD he had to make a declaration of tax compliance and provide a tax clearance certificate. If he cannot provide such a certificate today because of issues dating back to the 90s how could he properly do so in 2002? "
Accordingly, it is important that Mr Ahern's claim to have been tax compliant in 2002 should be independently reviewed," Mr Regan said today.
The complaint will increase the pressure on Mr Ahern and the Government but may not have any immediate effect.
The Taoiseach said in a recent radio interview that he believed the Revenue Commissioners would not make a final decision on his case until the Mahon Tribunal had finished hearing evidence from him and other relevant witnesses.
Sipo must then wait for Revenue to decide on whether to issue a clearance certificate and consider any appeal of its decision before proceeding.
In the event a certificate is not issued,Sipo will draw up a report for the Committee on Members' Interests of the Dáil. The Committee can publicly disclose the report by laying it before the Oireachtas and it can also recommend sanctions such as suspension or withholding of salary.
Sipo can also launch its own investigation because Mr Ahern is an office holder and can refer the matter to the Director of Public Prosecutions.
Former Fianna Fáil TD for West Limerick Michael Collins was the first TD to be prosecuted for obtaining a tax clearance certificate by false pretences in 2002. He was given a 12-month suspended jail sentence and a fine of €25,000.