Members of progressive Jewish communities are demanding a rabbis’ code of ethics following mounting concern that a Jewish religious leader who was accused of bullying and manipulative behaviour has been promoted to run one of the UK’s leading synagogues.

An open letter signed by scores of people demands that rabbis and cantors adhere to standards of behaviour, covering financial malpractice, bullying, harassment, inappropriate sexual behaviour and intellectual dishonesty. It says expulsion from clerical bodies should be available as a sanction for serious breaches of the code.

The letter follows dismay that the West London Synagogue (WLS), the flagship of the UK Reform Judaism movement, has appointed Rabbi David Mitchell as its joint senior rabbi to replace Baroness Julia Neuberger, who is retiring in March. Members of the synagogue’s congregation include historian Sir Simon Schama and the actor Maureen Lipman.

From 2012, complaints were made by some members of the synagogue’s staff of inappropriate behaviour by Mitchell – charges he denies. Several employees say they left their jobs as a result of bullying, derogatory comments and inappropriate references to sensitive personal information.

In a statement seen by the Guardian, one said that as a young person she had confided in Mitchell about her depression and self-harm. “He then used this confidential, private and deeply sensitive information against me when I was working at WLS as his subordinate,” she claimed.

Another complainant described a “toxic atmosphere with synagogue management seeking to quash rather than respond to any complaints”. She said she suffered from anxiety and depression and “cried in the toilets nearly every day for around six months”.

Individuals said they initially made complaints about Mitchell’s alleged behaviour internally. When it appeared no action had been taken, five made complaints in 2016 to Westminster council’s safeguarding unit.

In a letter responding to the allegations, the council said it had “taken your concerns exceptionally seriously and we are truly struck by your experiences” but that they did not warrant further investigation by the safeguarding unit.

It added: “WLS have reassured us they will be addressing the safeguarding claims.”

Jill Todd, the synagogue’s chair, invited the complainants to discuss their concerns, saying in an email that it would “help us to be assured that we are taking the appropriate action at West London Synagogue especially regarding your own personal concerns about David”.

However, ahead of scheduled meetings at WLS a few days later, the five say they were asked to sign non-disclosure agreements as a condition of being told the outcome of their complaints. The agreements committed them to keep the facts of the case and their own opinions “strictly confidential” or face the threat of legal action. None signed.

One complainant said they were subsequently told by WLS staff that Mitchell had been stripped of management responsibilities and removed as the synagogue’s director of education.

Another complainant said they were given assurances that he would not be appointed as senior rabbi. “We then left the matter alone, but now he has been promoted we are extremely worried,” said one of the five.

In a statement provided by WLS, Mitchell said: “I strenuously deny that I have acted inappropriately. I want to apologise for anything that I have done which has inadvertently hurt or angered others. Since these issues were first raised I have learned, with coaching and mentoring, to become a better manager of people and I am still learning.”

WLS confirmed that complaints were made in 2016 and said the synagogue was advised by Westminster council’s safeguarding team that they should be dealt with under internal management procedures.

“We took professional advice at the time to ensure that all our legal duties were discharged, all appropriate actions taken and that internal procedures were followed correctly. The complainants were notified of the outcome at the time,” it said in a statement.

It would be inappropriate to provide details of an individual case, it added, “although we can reconfirm that the appointment committee for the post of Senior rabbi was aware at the time of the appointment that historic complaints had previously been made about Rabbi David.”

One source told the Guardian there were “systemic problems” at the synagogue. “This is not just about David Mitchell, but about the culture at WLS,” they said.

The open letter, which does not directly refer to WLS, says all accusations of inappropriate behaviour must be taken seriously and investigated by an ethics committee of lay and professional leaders. A range of sanctions should be available, up to suspension and expulsion from clerical bodies.