A single official complaint regarding the behaviour of the deputy prime minister Dominic Raab represented the concerns of 27 of his officials, the Observer has been told. The group of Ministry of Justice officials are understood to be represented by a memo warning that some colleagues had been forced to take time off for “extended periods” as a result of having to deal with Raab. It states that others affected felt they needed to stay at work to stop extra pressure being placed on their colleagues. The complaint is among those being examined by Adam Tolley KC in an investigation into Raab’s behaviour during his ministerial career. The letter was first raised internally early last year, before being submitted as a complaint in November. It suggests the number of civil servants covered by complaints made to the inquiry is even larger than previously thought. “The combination of the pressure of work and unreasonable deadlines has had such an impact on some colleagues’ mental and physical health that they have visited their GPs, and some have subsequently been signed off work for extended periods of time,” the group complaint states. “Colleagues have confided … that they have been reluctant to be signed off due to the impact that this would have on their other team members.” The investigation into Raab represents a huge problem for Rishi Sunak, who has already had to sack the Tory chairman Nadhim Zahawi over a “serious breach” of the ministerial code, relating to his tax affairs. The Raab case, and the prime minister’s decision to appoint him to his cabinet, is another challenge to his vow to run a government characterised by “integrity, professionalism and accountability”. Raab denies bullying staff. It comes as a senior Whitehall figure with knowledge of the bullying investigation said it could take weeks or more to conclude because so much evidence has been presented against the deputy prime minister. They said Raab has not yet been interviewed as the evidence presented to Tolley mounts up. The source, who has discussed details of their complaints with some of the civil servants who say they were bullied, said the inquiry must not be rushed. “This man should be driven out of Whitehall never to return and that can only happen if this report concludes that. The integrity of this investigation is really important so he can’t cry foul and they cannot avoid the overwhelming evidence. Not only does Raab have to be sacked, but it is quite clear he can never be given responsibility again.” Senior Whitehall officials expect the inquiry to be thorough and have been impressed so far in their dealings with Tolley. However, there is mounting pressure in Whitehall for a change in the way complaints against ministers are handled, to stop them being ignored. “When it comes out how prolific he was, what a psychopath this man was, how he has destroyed lives and careers over three government departments where everyone knew – that is the moment when it will be clear that we can’t go back to the same system,” alleged the source. Insiders said senior civil servants and successive prime ministers had been made aware of Raab’s behaviour, only for him to be re-appointed by Sunak. The prime minister’s team has said he was not aware of any official complaints. Jake Berry, the former Tory chairman, said Raab should stand down while the investigation was complete. He said ministers were “not some form of special human being” and should be treated “like anyone else”. A Ministry of Justice spokesperson said: “There is zero tolerance for bullying across the civil service. The deputy prime minister leads a professional department, driving forward major reforms, where civil servants are valued and the level of ambition is high. There is an independent investigation under way and it would be inappropriate to comment further on issues relating to it until it is completed.”