The party has pencilled in an annual general meeting for some time around the middle of May when the handover of power - probably to Peter Robinson - will take place.

Senior DUP members hope it will prove a seamless transition, with Nigel Dodds becoming deputy leader and a reshuffle of the Executive team before the Assembly summer recess.

The senior figures argue a "new stamp" will then have been put on the Executive and Assembly by the time of the party conference - the first in more than two and a half years - in the autumn.

Insider sources today claimed Mr Paisley decided to resign soon after the result of the Dromore by-election, after considering his position for several weeks.

Support for Mr Paisley within the 36-strong Assembly group was dwindling, however, and, following a family discussion last week, it is understood he informed the party leadership he was standing down.

But reliable sources today also insisted the actual announcement of his departure still came sooner than they expected.

"It was thought he would do it next week, but he had his mind made up, and no one else made it up for him," one source said.

After the momentous events of yesterday, it was almost business as usual today as the outgoing First Minister conducted a full diary of engagements at Stormont.

He and Deputy First Minister Martin McGuinness will also make another of their joint public appearances tonight at the official opening of the Victoria Square retail complex in central Belfast.

They are due to be joined by the Social Development Minister Margaret Ritchie.

A spokesperson for the family said Mr Paisley, who has his 82nd birthday early next month, had a " variety of meetings" to attend.

In the weeks ahead, it is understood the now outgoing First Minister also envisages a major trip to Cork.

Although like Tony Blair he has announced his departure several months in advance, the DUP insists he will not be a "lame duck" First Minister.

"He knows there is a lot of business to be conducted and he intends in his normal way to go about doing it," one source said.

Paisley family members were today keeping a low profile as the implications and considerable political ripples from the announcement continued.

Although Mr Paisley did not openly endorse a successor, it is widely believed that Mr Robinson is the party favourite.

There was also speculation, however, that Mr Dodds could replace Mr Robinson as Finance Minister, and current Environment Minister Arlene Foster shift to Mr Dodd's current portfolio at the Department of Enterprise, Trade and Industry.

In a reshuffle, there could be promotion to Ministerial level for East Londonderry MP Gregory Campbell and Jeffrey Donaldson, only a week after he joined the Executive as Junior Minister.

Senior DUP sources have already said they hope there will not even be a contest for the new leader and deputy leader positions.

But there are tensions in other sections of the party which could resurface on the far side of the new Robinson regime.

One source today predicted: "It could be a fairly turbulent summer."

Mr Paisley has refused to give his endorsement to any individual to become his successor. Some in the party believe it could have caused more problems than it solved if he had.

According to one scenario, 59-year-old Mr Robinson would become First Minister for the rest of the term of the current Assembly and through the next election, retiring to leave the position to Mr Dodds if by that stage the DUP remains the largest party.