Kevin Keegan resigned as manager of Newcastle United on Thursday, less than eight months after taking charge of the Premier League club for the second time.
In a statement issued through the League Managers Association, Keegan said he had been left with "no choice" other than to leave because of the way the club was being run.
"I've been working desperately hard to find a way forward with the directors but sadly that has not proved possible," Keegan said in the statement.
"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."
Keegan's future at Newcastle had been in the spotlight since Tuesday when the club issued a statement saying he had not been sacked despite media speculation to the contrary.
Fans milled around outside St James' Park on Thursday after the news of Keegan's departure. Television pictures showed many looking stunned and close to tears while message boards were flooded with support for the former England midfielder.
Keegan, 57, who returned to Newcastle as manager in January, 11 years after his sudden departure from the job, was in charge for only 21 competitive matches this time, winning just six.
Newcastle have made a fair start to the season having won, drawn and lost in their opening three games to sit in mid-table with four points. They beat Championship (second division) side Coventry City 3-2 away in the League Cup second round last week.
However, the sale of promising winger James Milner to Aston Villa and a lack of major signings to excite the fans in the transfer window appear to have left Keegan feeling marginalised.
He was hailed as the club's saviour when he returned to St James' Park after they parted company with Sam Allardyce, who himself had only been in charge for half a season.
However, "King Kev" had a difficult start, failing to win any of his opening eight league matches.
More recently, some of Keegan's fans criticised his support of midfielder Joey Barton who he backed last month following the player's release from a prison sentence for assault.
With domestic action making way this weekend for a round of World Cup qualifiers, Newcastle's next match is against promoted Hull City at St James' Park on September 13.