Samaritans have abandoned the appointment of their new CEO after a Guardian investigation revealed claims that the Alzheimer’s Society paid out as much as £750,000 to staff who signed non-disclosure agreements (NDAs) during his tenure there.

Jeremy Hughes will no longer be taking the helm at the suicide prevention charity following whistleblower allegations that he displayed bullying behaviour to staff and presided over a toxic culture at the dementia charity.

The decision comes after two ex-Alzheimer’s Society employees who signed NDAs and received settlements after complaining of bullying and discrimination came forward on Monday to say Hughes should not take up the post at Samaritans, which he was due to start in May.

Earlier on Wednesday, the country’s biggest union, Unite – which has 50 members at Samaritans – joined calls for Hughes not to get the job. It said it was concerned about his appointment, claiming it had represented a number of members who suffered bullying at Samaritans by former members of its management team.

After trustees had been locked in talks for days, a Samaritans spokesperson said on Wednesday: “In light of events over the past week, the board of trustees has decided that it cannot proceed with the appointment of Jeremy Hughes as CEO, which was due to start in May. This decision is not in any way based on the allegations themselves, which Samaritans is not in a position to judge. It will begin a new CEO recruitment process in due course.”

Last week, the Guardian revealed the existence of an 11-page whistleblower complaint, made to the Charity Commission in February 2018, which warned the NDAs could be seen as an attempt to silence staff at the Alzheimer’s Society. Despite the serious nature of the allegation that hundreds of thousands of pounds had been spent to silence staff who signed NDAs, the watchdog did nothing to investigate the complaint – an oversight it now admits was a mistake.

Separately, a former senior staff member at the Alzheimer’s Society said Hughes had an “explosive temper” and displayed bullying behaviour towards staff, including shouting and undermining them.

Before Samaritans’ announcement, Unite regional officer Peter Storey said on Wednesday that the allegations about Hughes were “deeply troubling”.

“Given the fact that the Samaritans is in the very early stages of recovering from a toxic bullying culture which has damaged the lives of many of its staff, Unite does not believe that Mr Hughes is the right person to heal the problems the charity has experienced,” he said.

“Unite believes it is incumbent on the trustees at the Samaritans to rescind the appointment of Mr Hughes as its chief executive and instead appoint someone who has a track record in tackling a toxic bullying culture.”

The Alzheimer’s Society says the £750,000 figure for settlements is inaccurate. It declined to give its own sum. It said it only used settlements for “legitimate reasons”. The charity’s director of people and organisational development, Corinne Mills, said previously: “We are deeply concerned about the experience of the two ex-employees quoted. All complaints raised with us are taken very seriously and investigated thoroughly.”

She said the charity had “zero tolerance of bullying and discrimination – our evidence shows we have not and do not use settlement agreements or non-disclosure agreements to stop anyone reporting any whistleblowing, harassment or discrimination complaints”.

In an internal email sent by the charity’s chief operating officer, Kathryn Smith, to staff on Monday, Hughes – who is still in post as the society’s CEO – was quoted as saying: “I am deeply concerned by the allegations. I have always been determined to ensure everyone at the Alzheimer’s Society has a positive working experience. Everything I do is in order to make the biggest difference for people affected by dementia.”

• Footnote added on 30 May 2020: On 4 May 2020 the Charity Commission reported on the outcome of its regulatory case involving the Alzheimer’s Society, finding “no evidence that confidentiality clauses used by the charity were designed to or would have had the effect of preventing staff from reporting any whistleblowing, bullying, harassment or discrimination complaint. Allegations reported in the national media around the amount of money paid out in settlements were not substantiated by the evidence the Commission saw.” See press release here.

The article was also amended on 30 May 2020 to better reflect the terms in which the Alzheimer’s Society disputed the alleged figure for settlements.