The 57-year-old unilaterally announced his departure as manager after three days of talks with club's hierarchy.

And Wise's former number two at Elland Road Gus Poyet - first team coach at Tottenham - has emerged as the hot favourite to succeed Keegan at St James's Park.

Keegan released a statement last night evening through the League Managers' Association confirming his decision, which seemingly took the Magpies by surprise.

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The LMA revealed Keegan's move came after he received a letter from managing director Derek Llambias today which he felt left him with little alternative.

Keegan said: "I have been working desperately hard to find a way forward with the directors, but sadly that has not proved possible.

"It's my opinion that a manager must have the right to manage and that clubs should not impose upon any manager any player that he does not want.

"It remains my fervent wish to see Newcastle United do well in the future and I feel incredibly sorry for the players, staff and most importantly the supporters.

"I have been left with no choice other than to leave."

The move disappointed the club, who had spent the previous 48 hours trying to find a workable solution to a problem which had left angry supporters calling for the heads of owner billionaire Mike Ashley and executive director (football) Dennis Wise.

In a statement, the Magpies said: "Newcastle United Football Club is sad and disappointed that Kevin Keegan has resigned.

"Over the last few days, the club has devoted itself to the discussions it has held with Kevin and as a result of those discussions, had put together a set of practical suggestions for how to move forward.

"Today, the club made it clear to Kevin that if he had any outstanding concerns on its proposals, he should raise them with the club.

"The club regrets that Kevin has, instead of taking up that offer, chosen to resign."

The latest developments leave the Magpies looking for their sixth manager in four years, and facing the fury of fans who believe the man they consider a hero has been badly let down by Ashley.

Keegan was unhappy with the way the club conducted its transfer business over the final few days of he summer window, when it was claimed they tried to sell players behind his back while also failing to meet his expectations on signings.

Only Xisco and Ignacio Gonzalez arrived on Tyneside as the clock ticked down, and neither are understood to have been on the manager's wish-list.

He made his feelings abundantly clear at meetings with the board on both Monday and Tuesday as rumours spread that he had been sacked or had walked out.

But after a deafening silence from St James' Park on Monday, the Magpies issued two statements either side of another from the man himself revealing that concerns had been raised and were being discussed.

Keegan, Newcastle insisted, had neither resigned nor been sacked, and the ongoing dialogue was aimed at resolving the differences.

However, even as the talks went on, the word coming out of the Keegan camp was that it was only a matter of time before his second spell as manager came to an end.

All the talk was that he would not resign because of the £1million it would cost him to get out of the three-and-a-half year contract he signed just eight months ago.

But Llambias' letter appears to have made up his mind, ironically just a day after West Ham counterpart Alan Curbishley left his post citing similar reasons.

There were angry protests on Gallowgate at the prospect of the adopted Geordie leaving he club for a third time after his spell as a player and then as manager between 1992 and 1997, and that fury will only intensify now that his departure has been confirmed.

Ashley and Wise, the man he appointed to head up the club's recruitment arm, bore the brunt of the supporters' jeers, and it is difficult to see how they can be appeased.

The club can expect concerted protests, especially when the Magpies return to Barclays Premier League action against Hull on September 13.

In the meantime, Ashley will once again begin the process of finding the man to lead the club forward in a job which seems to resemble the proverbial poisoned chalice ever more by the year.

Wise was swiftly installed as the bookmakers' favourite on Monday, and he would certainly be the cheap and convenient option, although just how well that would go down with players who were hugely loyal to Keegan is a moot point.

David O'Leary and Everton's David Moyes have also been mentioned as possibilities, and so too has Real Zaragosa's Marcelino Garcia Toral, but the list is bound to grow by the hour in the aftermath of another St James' debacle.

But Ashley's long-term task will be to persuade fans who expected major investment when the billionaire bought out Freddy Shepherd just last year that his vision for the club is the right one with the Magpies seemingly having been left behind by the Premier League's other new owners.