Civil servants who accused Dominic Raab of bullying them have said they would not go through the process again because it is “flawed”, the head of the civil service union has said.

Mr Raab resigned from the Cabinet in April after an independent investigation found he had bullied staff engaged in an “abuse or misuse of power” that “undermines or humiliates” while foreign secretary.

The five-month investigation by lawyer Adam Tolley was launched after multiple official complaints were made against the then-deputy prime minister, but Dave Penman, the general secretary of the FDA, told the PA news agency on Thursday that Mr Raab’s accusers would not recommend complaining again.

He told PA: “I spoke to some of the people who raised complaints, and they’ve said: ‘We’d never do it again, we would not go through this again.’

“I just simply wouldn’t advise anyone, because the process is so completely flawed and one-sided, I wouldn’t advise anyone to go through this again.”

Mr Penman also criticised Rishi Sunak’s handling of the report into Mr Raab’s conduct as “appalling”, saying it had “sent an absolutely clear message to civil servants whose side he’s going to be on”.

I just simply wouldn’t advise anyone, because the process is so completely flawed and one-sided, I wouldn’t advise anyone to go through this again.

He said: “Every civil servant who was brave enough to come up and raise a complaint, whether it was found against or not – and Tolley said all of the complaints were made in good faith – had to sit at home and wait and watch this unfold on telly.

“They weren’t told what was happening, they’ve no rights in the process, and the Prime Minister allowed Dominic Raab to go in there and try and restore his reputation, and then sent him a letter that looked like he was retiring to be with his family, rather than a minister who had been found to be bullying civil servants in breach of the ministerial code.”

In his response to Mr Raab’s resignation letter, the Prime Minister acknowledged he had “rightly” promised to resign if a finding of bullying was made against him, but praised his “record of delivery” and said there had been “shortcomings” in the investigation’s process that had “negatively affected everyone involved”.

Mr Raab also gave an interview to the BBC in which he claimed “activist civil servants” were “effectively trying to block government”.

The reaction to the findings of bullying against Mr Raab led Hannah White, director of the Institute for Government, to warn that “no civil servant would feel encouraged to speak out in future”.

Downing Street has already pledged to learn lessons from the investigation about how bullying claims are handled in government.

On Thursday, Mr Penman used his speech to the FDA conference to criticise ministers for “undermining” the civil service and call on union members to vote in favour of industrial action after the Government refused to offer a cost-of-living payment to civil servants.

He told PA: “If you’re constantly going to treat people like that, then what ends up happening is some things just tip the balance.

“It’s about being called woke, it’s being accused of being ‘the blob’, and now you’re doing this about pay as well, and I think that’s the difficulty.

“Once you do that and you essentially say you’re not valued across a whole range of measures, that’s when people decide to go.”