The finger of suspicion and the taint of corruption may have lingered over senior police officers, played by Gina McKee and Pippa Haywood, in BBC One’s hit drama Bodyguard – but the police adviser of the show insists it has not damaged the service’s reputation.

As followers of the show, watched by 11 million viewers, wait to find out who is unmasked as the villain on Sunday night in a final 75-minute episode, former Metropolitan police commander David Zinzan told the Observer he wanted to reassure the public that corruption was not widespread.

“It’s true that a television series can influence people’s perceptions. I would like to hope the audience will see the bigger picture though,” said Zinzan, who served on an anti-corruption squad in the Met. “I wouldn’t want them to think most cops are corrupt. I came across a small number of officers and it was sad to see even that. You always get a few who are, but that is probably true in any profession.”

Zinzan spent 30 years at Scotland Yard working on counter-terrorism, homicide, serious and organised crime, as well as on anti-corruption, and also served as deputy chief constable of Devon and Cornwall. “I have also worked at a senior level with colleagues from security services, and I found them to be very professional and straight,” he said. “In my view it is healthy that in this country the BBC is allowed to make a programme that questions the integrity of these organisations.”

Zinzan worked closely with the show’s creator, Jed Mercurio, and has also advised on seasons three and four of the writer’s acclaimed police drama, Line of Duty, as well as on its fifth season, now being filmed.

Gina McKee as the steely Anne Sampson, head of the Met’s counter-terrorism command. Photograph: Sophie Mutevelian/PA

On Bodyguard, which ranks alongside Sherlock and Call the Midwife as one of the decade’s top 10 television dramas for audience size, Zinzan worked with an advisory team that included a former “close protection” officer and a bomb disposal expert.

“Jed would pass me the scenes as he wrote them,” said Zinzan. “There’s always a balance to be struck between total authenticity and drama, and I help to bridge that gap. I advise on correct language and terminology and uniforms. On set, I’m also a resource for the actors if they want to talk about their mindset before a particular scene. If anything doesn’t look right, Jed wants me to say so: on the other hand, if it was totally authentic it would not be so thrilling.”

The sexual affair between the home secretary, played by Keeley Hawes, and her protection officer, actor Richard Madden, was “completely taboo from a police perspective”, said Zinzan. “Any working relationship where people work side by side will have inappropriate relationships forming, I realise,” he said. “But you would be removed from duties if it happened. One of the first rules is you don’t form that kind of attachment.”

Zinzan conceded that the protection officer who looked after Labour home secretary Alan Johnson a decade ago was revealed to have had an affair with Johnson’s former wife.

“The Met took a very dim view of that,” said Zinzan. “The officer let the side down and his team.”

Madden’s portrayal of the battle-traumatised David Budd occasionally went close to the line of plausibility, Zinzan agreed, in order to keep viewers guessing.

“A close protection officer would work on shifts and hand over after 12 hours. They want you to keep fresh because potentially you have to use lethal weapons,” he said. Any symptoms of PTSD would also result in the loss of a firearms licence in real life. “This is a crime drama and part of the appeal is the tension. But few officers handle firearms, and those that do are subjected to vigorous testing. That is why we have fewer incidents of cops running amok than in other countries.”