Against a backdrop of mounting unease within DUP ranks, however, Mr Paisley had come under increasing pressure and criticism in recent weeks.

Yet, even senior party members had not anticipated his decision would come so soon.

The veteran political firebrand who will be 82 next month, confirmed he will stand down as both First Minister and party leader " shortly after" the International Economic Conference in May.

" When I make a break, it is a break," he quipped last night.

It is likely he will be succeeded in both roles by his long-serving party deputy, Peter Robinson.

He will be leaving, however, after just over a year in office, when he had clearly signalled his intention to serve the full four-year term.

Mr Paisley said he had reached his decision a few weeks ago, around the time his son, Ian Paisley jnr, resigned as Junior Minister after months of serious allegations over his links with property developer Seymour Sweeney.

Yet it was the unexpected loss of the Dromore by-election which brought growing DUP disquiet to the boil.

But the First Minister stoutly denied the claims had lead to his decision to go, and argued people had tried to "get to" him through his son.

"I am not a fool, people who thought that they could get at me, got at him," he said.

"They thought they could damage me by the damage they sought to take out on him, but that did not move me."

Instead, he said, he had come to realise that the massive Economic Conference planned for early May, the first anniversary of the new regime at Stormont, would be a fitting time to bow out.

"I thought that it is a marker, a very big marker and it would be a very appropriate time for me to bow out," the DUP founder added.

Though he had a personal scare several years ago, which was said to have been related to medication, it is not believed health is a major factor in his verdict which party sources said he had reached 'on his own'.

Mr Paisley, having decided the time was right to go into government with Sinn Fein, brought a high level of commitment to his job and had not shown signs of weariness despite the tensions of recent weeks.

He had, however, been sounded out about his views for the inevitable, eventual succession and bringing about as smooth a transition, both at Stormont and in the party, as possible.

But he refused to name his successor last night in an interview with Ulster Television's Political Editor Ken Reid.

"This is not the Church of Rome. This is not Apostolic succession and I have no right to say who will succeed me," he said.

Tributes were immediate and from across the political spectrum from Prime Minister Gordon Brown, with whom Mr Paisley tussled over the 'economic package' for the province, and his deputy coalition partner, Martin McGuinness.

Under the party's rulebook, any candidates for the leadership of the party will first face a ballot of the 26-strong Assembly Group, which will then make a recommendation to the party executive.

It would be unlikely, however, that the executive would overturn the choice of the Assembly group. Unease within the party is believed to have grown in the aftermath of confirmation at the weekend that Ian jnr has been appointed to the Policing Board.

Some colleagues believed it was too soon for the North Antrim MLA to be given a high profile post and his comments on being appointed, to the effect that he would take on Sinn Fein, were an implicit criticism of colleagues already serving on the board.

For a few, it was the final straw. After a flurry of meetings, Mr Paisley hastily organised a series of media interviews yesterday to make clear his intentions.