Norman Smurthwaite has stood down as Port Vale chairman, saying he has "seriously damaged the club", and has been succeeded by Tony Fradley.
Vale's four-year spell in League One ended with relegation on Sunday.
The 56-year-old, who first became involved with Vale in 2012 and been chairman at Vale Park since 2013, put the club up for sale in December 2015.
Fradley, the club's president since 2014, said it was an "honour" to take on the role.
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Smurthwaite, who has also said he will not attend any future matches at Vale Park, was part of the investment group that took Vale out of administration in November 2012 and he became chairman the following year following Paul Wildes' resignation.
Vale made a positive start under their new Portuguese boss, but results dipped before Ribeiro resigned in December and caretaker boss Michael Brown was unable to keep Vale in the third tier.
Relegation to League Two was confirmed on the final day of the season, with Vale drawing 0-0 at Fleetwood and failing to secure the win they needed for safety.
"I have clearly seriously damaged our club, resulting in Sunday's relegation which was never my intention," said Smurthwaite in a statement.
"I deeply regret the actions and appointments I made during the  closed season and would like to apologise sincerely to all supporters, Port Vale staff and the wider Burslem community for the impact of my decisions last summer."
He continued: "I have had a modicum of success in my business life and when I endured setbacks, of which there were many, I used to dust myself down and get back into the project in hand.
"Sadly, due the environment of the last few months, I do not feel that is now possible.
"It is with a heavy heart that I have decided to stand down as chairman with immediate effect and relinquish all remaining duties that I am involved in.
"I will leave it to (chief executive) Colin Garlick, and the team he is building, to work through what will be a very difficult period for the football club."
Garlick said Smurthwaite's decision was "very difficult for him", saying that he was "hurting badly".
He told BBC Radio Stoke: "He (Smurthwaite) has taken responsibility on board. You can see how heartfelt the statement he has made is. He believes it's the right thing to do."
Garlick added that Vale plan to make an announcement about a full-time manager this week, with Brown saying he hoped to be given the job despite Vale's relegation.
Fradley, 67, said: "It's an honour and a privilege to be appointed as chairman.
"I've been a supporter since the 1950s, so I'm totally committed to the long-term success of the club to get it to where we want to be."
Lee Blakeman, Port Vale commentator for BBC Radio Stoke
I don't think Smurthwaite's departure comes as a great surprise, considering what happened last summer. He made all the decisions with player contracts and cutting the playing budget, and admitted he got them wrong.
He appointed Ribeiro, the club's first foreign manager, and then hired Brown as caretaker boss. Even after all that, they still went down.
All the decisions he made went against him, which in turn turned the fans against him.
I'm loathed to say his position was untenable, but, considering the relationship he had with supporters, it really was impossible for him to stay.