The spectre of environmental ‘domestic extremism’ has long been postulated by police leaders and security analysts in Britain. It is a narrative that has justified the commitment of enormous amounts of government resources towards police intelligence work directed at non-violent direct action campaigns. Most controversially, this has included the long-term infiltration of environmental (and other) activist groups by undercover police. This article provides a critical analysis of the justifications put forward in support of the covert surveillance of environmental activists in Britain. The paper proceeds by way of a single case study – a high profile, environmental direct action protest in the north of England – in order to reveal the levels of abuse, manipulation and deception at the basis of undercover protest policing. Through their court case, the activists involved with this action were able to obtain rare insights into the police authorisation documents for the undercover operation that had led to their arrests. An analysis of these documents provides us with a glimpse of the contradictory justifications given by senior police officers for infiltration – now under scrutiny by a public inquiry.