Peter Lawwell spent 18 years as chief executive and three as chairman will be replaced with interim chairman Brian Wilson.
Peter Lawwell is standing down as Celtic chairman at the end of December, citing “abuse and threats from some sources” he finds “intolerable”, according to BBC Scotland.
The Scottish champions’ board have been the subject of fan protests this season following a disappointing summer transfer window, their exit from the Champions League and with the current side sitting behind Heart of Midlothian in the Scottish Premiership.
New manager Wilfried Nancy has also lost his first three games in charge after succeeding Brendan Rodgers, who resigned in October, including Sunday’s Premier Sports Cup final against St Mirren.
Non-executive director Brian Wilson will become interim chairman from 31 December until a permanent appointment is made.
Lawwell, who became Celtic chief executive for 18 years from 2003, told the club website it was “with sadness” that he was ending his three-year spell as chairman.
“I believe that my 18 years as chief executive and three years as chairman at the Club have shown my ability to meet and overcome challenges on many fronts, but abuse and threats from some sources have increased and are now intolerable,” he said.
Meanwhile, Rangers have appointed Stig Inge Bjornebye as football consultant and advisor for an initial period of around six months.
The former Blackburn Rovers, Liverpool and Norway player starts immediately and will report to chairman Andrew Cavenagh.
Bjornebye, who has also held the Norway assistant and IK Start head coach roles, worked as a sporting director at Rosenborg, where he also played, and Danish club Aarhus.
Rangers continue to look for a sporting director following Kevin Thelwell’s departure. Chief executive Patrick Stewart left at the same time and Fraser Thornton is covering that role on an interim basis.
The Ibrox side, who beat Hibernian 1-0 on Monday, are third in the Premiership.
Danny Rohl’s side visit leaders Hearts on Sunday (13:30 GMT) and have two further games at home before the January transfer window opens, with Bjornebye expected to play a role in transfer dealings.