The political fortunes of Bertie Ahern, the most successful Irish leader since independence from Britain, have fallen to a low point after he admitted that his tax affairs were not in order.

Mr Ahern, who has won three successive general elections — the last in spite of damaging revelations about his personal finances while finance minister in the 1990s — is already being talked about as a “lame duck” in spite of having more than four years left in office.

His current woes are linked to a long-running public investigation, the Mahon tribunal, into land planning deals linked to political favours. The Taioseach has undergone days of punishing cross-questioning about so-called “dig-outs” — loans or gifts from rich businessmen.

The tribunal is investigating claims by a developer, Owen O'Callaghan, that he gave substantial sums of cash to Mr Ahern between 1989 and 1992, and a series of payments he received when he was finance minister in the 1990s.

The Taoiseach has consistently maintained his innocence. “Mr O'Callaghan never gave me a penny. Luckily from my point of view, he never bought me a glass of water,” he said.

Even though his personal ratings have suffered, the public has been prepared to back the Fianna Fail leader while the tribunal failed to land any knockout punches during examination of his financial arrangements. Among the disclosures was that Mr Ahern cashed his government salary cheques in his local pub.

Mr Ahern has now admitted that he will not be able to provide the state's Standards in Public Office Commission (Sipo) with a certificate proving that he is tax compliant when a deadline for newly elected members of the Irish Parliament, the Oireachtas, expires next month.

The commission was established in the mid-1990s amid concern about tax avoidance in high places. Oireachtas members are required to provide it with either a tax clearance certificate or an application statement within nine months of a general election taking place.

The latter is a statement from the Revenue Commissioners - the state tax authority - stating that an individual has applied for a tax clearance certificate but that a decision on the application has yet to be made.

Mr Ahern has said: “I have made a statutory declaration under Section 21(1)(b) of the Standards in Public Office Act to the effect that I was - and I am - to the best of my knowledge and belief, in compliance with my tax obligations. That is my position. That will, I am confident, be evidenced in due course by the issue of a tax clearance certificate.”

He also acknowledged that he had made a preliminary payment to the Revenue - reported to be some 70,000 euros (£52,000) — but said that this was normal practice and “does not necessarily mean there is a liability arising”.

Mr Ahern accompanied the admission — made after letters exchanged between him and the Revenue authorities were leaked to the press — with his strongest attack to date on the Mahon tribunal, accusing it of being the source. “I think it is a new low in Irish life, not Irish politics, that Revenue letters are to be leaked,” he said, adding that some of the questions the tribunal had posed him “went beyond the bounds of common decency”.

He said that questions about his private life were “prying and prurient”.

Opposition parties claimed that his attack was a “desperate attempt” to divert attention from the questions over his tax affairs.

Fine Gael accused Mr Ahern of deliberately misleading the Dáil — the Irish Parliament — when he told it in 2006 that his tax affairs were in order.

Joan Burton, deputy leader of the Irish Labour Party, said that it was extraordinary for a serving Taoiseach to admit that he did not have full tax clearance. “Is there any parliamentary democracy in the world where a head of government who found himself in this situation would be allowed to remain in office?” she asked.

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