How undercover police turn their worst nightmares into reality

G.K. Chesterton's novel The Man Who Was Thursday involves an anarchist cell, every one of whose members is revealed to be an undercover policeman.

I have been thinking of it in recent days, as the number of undercover police officers involved in environmental pressure groups has become clear. Because it seems to me that the police are in danger of turning their worst nightmares into concrete reality.

First they convince themselves that essentially herbivorous groups like Plane Stupid or Reclaim the Streets are plotting to disrupt the nation. So they send undercover officers to join those groups and take the lead in plotting just the sort of destructive actions that they fear.

Then in today's Observer an undercover officer is quoted as saying: "You cannot not be promiscuous in those groups. Otherwise you'll stand out straightaway."

Well, maybe. But maybe too the promiscuity of environmental activists is a figment of police imagination. And maybe by attempting to fit in with this imagined promiscuity they make it a reality - at least in their own lives.