Mr Sadler lost his job just before Christmas with his college facing publication of its Ofsted inspection and a strike by lecturers who were angry that he had sacked their union branch chairman, Hugh Hobson.
Mr Hobson, who represented the University and College Union, had been dismissed after allegations of gross misconduct, prompting him to lodge a grievance that he had been unfairly targeted because of his union activities.
The Association of College Managers, representing Mr Sadler in his appeal against dismissal, said the principal was suspended after complaints from staff. It said the complaints were not upheld.
The ACM has complained that Mr Sadler was then dismissed by a meeting of the board of governors called at two days' notice and that some governors claim not to have received notification of the meeting.
Peter Pendle, general secretary of the ACM, said: "The articles of governance are clear on the process to be used when dismissing a senior postholder and we are concerned that these may not have been followed. At no stage has Peter been given an opportunity to state his case or answer the allegations made against him."
In an email to college staff dated January 6, Roger Neville, the chairman of governors, said: "As with all staff employed by Epping Forest College, there is provision for a review towards the end of the initial probation period. The board of governors carried out such a review just before Christmas and has concluded that Peter Sadler will not be confirmed in post."
Mr Neville called on staff to rally behind the senior management team, which will be led by interim principal Jeannie Wright.
No date has been fixed for the strike. Barry Lovejoy, UCU's head of colleges, said they now hoped to work with Ms Wright to resolve their differences. The college said it was still in talks regarding Mr Sadler's future and could not comment further for legal reasons.
Epping College's Ofsted report is due to be published next month.