It is striking how a significant number of undercover officers have experienced mental breakdowns because of the pressures of their job.

This is a subject which we are going to examine closely in the book. We would of course been interested to hear from anyone who has information on this.

It is clear that undercover officers who have infiltrated political groups have had mental breakdowns.

Peter Black, a Special Demonstration Squad (SDS) officer who was sent to infiltrated anti-racist groups in the 1990s, has described how he was diagnosed as suffering from post-traumatic stress disorder. He sued the Metropolitan Police and received an out-of-court settelment.

He described the strain of living a double life for years in an interview in 2010.

We understand that there are others. The problem for undercover officers was dealing, for instance, with stress over a prolonged period as well as the conflict of living out two identities, their real one and then their fake one.

There's however a crucial question here - did the police chiefs who were responsible for deploying undercover officers in political groups do enough to ensure the mental well-being of those officers?

For instance, did those running the Special Demonstration Squad and the National Public Order Intelligence Unit (NPOIU) ensure that their officers were able to go regularly to psychologists to assess their mental state so that problems could be picked up early? Did the police chiefs learn from previous cases and improve their procedures?

Using the freedom of information act, we asked Scotland Yard for copies of policies which were in place after 2000 to ensure the psychological health of undercover officers in the SDS and NPOIU.

Scotland Yard has refused to release any documents. The Met argues that disclosing the documents could "damage future police investigations and hinder the way in which the NPOIU can operate in the future".

The Met adds that "the publication of information that could reveal operational methodology could have a prejudical impact on the investigation of crime generally".

We disagree and will be appealing the decision. We will argue that the documents do not contain information which is operationally sensitive and that moreover the targets of the infiltration would be unable to deduce from the documents information about tradecraft which would compromise operations.

Disclosure of the documents could help shed more light on a claim by Mark Kennedy, the undercover officer deployed by the NPOIU to infiltrate the environmental movement for seven years. He has said :"I was supposed to get psychological counselling every three months. I would go two years without seeing the shrink. Initially meetings were regular. Then it became a farce. The office was so greedy for intelligence that they didn't set up the meetings. They went by the wayside. I'm sure that's the same for other undercover officers too.'

It is also evident that officers who have gone undercover to infiltrate serious criminals such as drug gangs have also gone off the rails. For example, Liam Thomas memorably described himself as a "broken biscuit" when he left the Metropolitan Police on medical grounds in 2004. He had spent a decade deployed across Britain in long-term infiltrations. Last year, he said :"The vicious internal politics and pressures of my undercover life led to a suicide attempt and mental breakdown".