The Cops, Tony Garnett's Bafta award-winning drama about rank and file police officers begins its second series on Monday (9pm, BBC2). Like the first series it is billed as a faithful warts-and-all revelation of life on the beat. But this time around, it is produced entirely without police assistance.

After seeing trailers for the first series, the Greater Manchester and Lancashire Police forces decided that the journey through the mind and soul of the modern bobby ventured too close to the occasionally rotten heart of our boys and girls in blue. Both forces withdrew their assistance and refused to advise or co-operate in any way on the second. Tony Garnett, who previously made This Life, Cardiac Arrest and Cathy Come Home, is not surprised at the reaction. "All cop shows except The Cops are good PR for the police. But I am not in the PR business."

Programme makers and their institutional subjects have often been uneasy bed-fellows. The danger of sleeping with the enemy was experienced by Psychos, Channel 4's critically-acclaimed drama set in a mental hospital. The producers worked with three psychiatrists and had access to the Maudsley Hospital, but were kept out of all Scottish institutions (the fictional hospital of the series was set in Glasgow). Mind, the mental health organisation, was consulted during production, but mounted a campaign in opposition to the programme's title. Kudos, the producers, and Channel 4 chief executive Michael Jackson were inundated with complaints.

Executive producer Stephen Garrett says he received 50 letters a day at its peak, "but they were all from the same postal address, so it didn't have quite the impact it could have done". The experience was, he says, "mildly irritating", and the producers and Channel 4 saw it for what it was: "political correctness run rampant". He adds: "If you try and tell a broadcaster like Channel 4 to change the title of a show, they will go in the opposite direction, as quite rightly, they cannot be seen to bow to that sort of public pressure."

But many producers claim a certain amount of bowing and scraping to officialdom is essential to satisfy the modern audience's demand for credibility and realism. Bomber, an ITV drama starring Mark Strong and launching in January 2000, gained unprecedented access to the army's secretive Explosive Ordnance Disposal Officers, or EDOs. The producers, Zenith, say they could not confidently have gone ahead without the Army's assistance.

Archie Tate, producer of Bomber, said he wanted "to put the show on an official footing as soon as possible. EDOs are inexplicably heroic, and we didn't feel there was anything to be gained by not getting full co-operation from the army". But in return for their assistance, the army could veto certain story lines and insisted on seeing a full script before granting approval.

Tate recalls a scene featuring a drunk army officer vomiting over a war memorial. Unsurprisingly, the army took exception. "We looked at it again and came to the same conclusion," says Tate. "It was over the top and gratuitous." The scene was cut.

Psychos' Stephen Garrett, however, reacts with disdain at allowing any outsider editorial decisions or script approval. "It's the kiss of death if a producer has to get approval from the group they are portraying," he says. "I would have seen it as a huge admission of failure if the mental health establishment had said 'Psychos is a wonderful portrayal, it's just what we want to tell people'."

Institutions have become accustomed to an easy ride with long-running primetime series. Shows such as The Bill and London's Burning rely on a harmonious, some would say cosy, relationship with the police and fire service. The Bill, often criticised as a "mouthpiece for the Met" is "very popular" with the police who have not issued a single complaint in 15 years, says executive producer Richard Hanford. But he is sensitive to accusations of the show being propagandist. "We simply aim to be the most realistic police series on TV," he says.

Hanford is often informed of "new initiatives" that the Met is developing which may then be crafted into scripts. "I don't see that as cosy, I see that as privileged access to greater realism for our story lines and a better show for viewers," says Hanford.

Jackie Malton, an ex-detective who was the inspiration for Helen Mirren' s character in Prime Suspect and continues to work closely with Lynda La Plante, advises on The Bill. She sees everything at treatment stage but is adamant that drama comes first. She says The Cops has "stretched the genre to extremes" and believes the Greater Manchester Police feel "they had had their fingers burnt".

Following the proliferation of documentary dramas, producers of fictional drama feel under unprecedented pressure to steep their shows in realism. Stephen Garrett has found the traditional line dividing drama and documentary blurring as audiences and institutions come to expect a more faithful representation on screen, oblivious that the two genres are very different. "Drama producers are not there to document what life is like in institutions, we are there to entertain," he says. "Let's face it, if real emergency rooms were actually like Casualty, the whole world would fall apart."