After 11 years as Ireland's leader, Bertie Ahern yesterday was forced to announce his resignation as a scandal about his personal finances deepened. He will step down as taoiseach and leader of Fianna Fail on May 6.

Pressure had been building on the 56-year-old from within his party, with colleagues fearing claims about his financial affairs could cause it serious electoral damage.

Yet yesterday morning Mr Ahern caught the public by surprise when he called a snap news conference on the steps of the Government Buildings in Dublin.

In an emotional statement, he professed his innocence of any wrongdoing as a tribunal in the Irish capital continued to look into allegations of planning corruption and payments by businessmen to politicians. It has been doing so for 11 years but recently has focused on Mr Ahern's finances.

Referring to his role in the Good Friday Agreement and in Northern Ireland's peace process, the taoiseach insisted he had not taken a bribe. He said: "The priority I put above all others was to work for peace on this island and I have given all to that cause. I know in my heart of hearts I have done no wrong and wronged no one."

He stressed how his decision to resign was a personal decision motivated by what was best for the people of Ireland and that he would not allow personal issues to dominate the nation's politics.

He added: "While I will be the first to admit that I've made mistakes in my life and in my career, one mistake I've never made was to enrich myself by misusing the trust of the people. I have never received a corrupt payment and I've never done anything to dishonour any office that I've ever held."

Standing next to Mr Ahern was his grim-faced deputy Brian Cowen, the Finance Minister, tipped as the favourite to succeed as taoiseach.

Mr Ahern's resignation comes as voters in the Irish Republic learned that they will vote on the Lisbon Treaty, aimed at smoothing the working of the EU, on June 12. But the instability caused to Fianna Fail could affect the vote and if the Irish were to vote against it, then the entire treaty would fall, plunging Europe into crisis.

Last night, the sequence of events which led to Mr Ahern's resignation were being pieced together. Last Thursday, he met Mr Cowen for the first time since Grainne Carruth, his former secretary, blatantly contradicted his earlier evidence to the Mahon Tribunal about the depositing of pounds sterling.

Sources said both men agreed a departure strategy was needed and they began discussions on the announcement of a date for Mr Ahern to leave. In the ensuing days, he pulled out of public engagements. Colleagues tried to defend him but it was clear they had no knowledge of how their leader would explain the clash of evidence.

Enda Kenny, the Leader of the Opposition Fine Gael, claimed the taoiseach had bowed to the inevitable and called on his successor to seek a mandate in a fresh election.

Tributes poured in for Mr Ahern, focusing on his achievements in the Northern Ireland peace process and in Europe.

Mary McAleese, the Irish President, described his contribution to all of these as "hugely important." Gordon Brown, the Prime Minister, praised his "historic contribution in helping to bring peace to Northern Ireland, transforming Ireland's relationship with the UK and playing a key role in the development of a forward looking and dynamic Europe".

Alex Salmond, the First Minister, noted how the "accomplishments of the Irish Celtic tiger economy demonstrate the advantage of being a small independent nation in Europe". Former PM Tony Blair, who worked with Mr Ahern on the peace process, said: "He will always be remembered for his crucial role in bringing about peace in Northern Ireland."

Gerry Adams, the Sinn Fein president, added: "He is justifiably proud of the fact that he was the taoiseach who was in place and who, with the British Prime Minister and the rest of us, brought about the Good Friday Agreement and he deserves our thanks for that."

Ian Paisley, Stormont's First Minister, said: "In sharp contrast with other Irish Prime Ministers, I enjoyed a good working relationship with Mr Ahern. He was willing to recognise the Unionist population had no interest in being part of a united Ireland."