Kent Police have belatedly admitted they planted a Special Branch detective inside a public meeting to monitor the activities of environmentalists.

Initially, the force would only confirm that a chief inspector from Dover had attended the debate at Canterbury Christ Church University in November as any exploratory drilling is most likely to take place in his area.

It prompted police and crime commissioner Ann Barnes to order an investigation into the police presence.

Now, a report by Deputy Chief Constable Paul Brandon admits that a Special Branch detective was there for “the gathering of information and intelligence that could prevent harm being caused to those attending such a meeting or any similar meetings” and was involved in Operation Keswick, the Force’s response to fracking protests.

Called Fracking in the UK, the meeting took place on November 19 and featured both those in favour and against drilling for shale gas.

"Public debate on important issues should not be subject to police surveillance and to learn that Special Branch were there is frankly appalling..." - Stuart Jeffery

Among the speakers were prominent Green Party activists and anti-fracking campaigners such as Julie Wassmer from Whitstable.

When it emerged officers had attended, Kent Police was accused of an “abuse of power” and “political policing against the Greens”.

This week Stuart Jeffery, the Green Party general election candidate for Canterbury and Whitstable, said: “It seems the level of police anxiety over a simple public meeting was even higher than previously thought.

“They had already admitted that a chief inspector has attended the meeting in plain clothes and now we learn that a Special Branch officer was planted in the meeting as well.

“Public debate on important issues should not be subject to police surveillance and to learn that Special Branch were there and that their spying operation was given the name Operation Keswick is frankly appalling.

“I attended the meeting as an interested member of the public and as someone who has a deep interest in fracking.

“While I would not be put off by the police’s actions, I am certain that others would.

“It is important people are able to debate important issues freely. There were no demonstration at the meeting and no public disorder.

“It is clear the police have overstepped the mark.”

Mr Brandon found that a complaint from Mr Jeffery about police attendance at the meeting was unproven.

But he said officers involved would receive words of advice in order to be “clear and specific” about their work so that they could “avoid ambiguity and misunderstanding” when dealing with outside organisations.