A team of Greenpeace activists hacked in to and intercepted live CCTV footage from a police helicopter during a protest at a power station, to use in a documentary released this week.

Police are investigating how the activists managed to obtain footage beamed from their helicopter to officers on the ground during an invasion of the coal-fired Kingsnorth power station near Rochester in Kent, by Greenpeace members in 2007.

The footage has come to light only this week with the release of the documentary — made free of charge by Nick Broomfield, the Bafta award-winning film-maker — which uses parts of the helicopter footage to show the activists at the top of the 200-metre power station chimney.

Tim Hewke, who co-ordinated the 2007 protest, was reluctant to reveal exactly how Greenpeace intercepted it and said: “Someone, somewhere has a piece of kit that’s used from time to time.”

John Sauven, head of Greenpeace in Britain, told The Times: “I don’t know exactly how they did it, but the equipment is ours, and I’m assuming it’s not that difficult to do — to pick it up like you can pick up television or radio waves.

“The police film is of us not of them, so I think it’s only fair we have a copy. We have surveillance by the police photographing our members as they come into the office some days, so it’s nice to turn the tables on them and do the opposite.”

The protest at Kingsnorth coal-fired power station took place in October 2007, when 50 Greenpeace members gained access to the site and scaled the chimney, intending to paint the words “Gordon bin it” on the outside in protest at plans to build a new coal plant on the site.

Six of the activists were arrested at the scene and stood trial last year. Though they admitted to causing £30,000 criminal damage, all six were acquitted after expert evidence that climate change was a serious enough threat to have justified the activists’ actions.

Mr Broomfield’s film, A Time Comes, follows a group of activists as they gain entry to the power station while a staged protest at the front gates creates a diversion. The group is then showing scaling the inside of the chimney and climbing out on top, where they set about cooking a meal and painting their message on the outside of the structure.

The film cuts between face-to-face interviews with the activists, footage taken on hand-held cameras by the protestors and the pictures from the police cameras, which were downloaded on to a hard-drive and given to Mr Broomfield.

He said: “I had no reservations in using the police footage. I felt that it was all part of a very valid protest. I didn’t feel that it compromised anyone’s safety, so I didn’t hesitate.

“It’s very sexy footage. I have to compliment the police on their photography. I don’t know how the Greenpeace guys did it, but they managed to intercept the signal without stopping the police from seeing the pictures.”

He dismissed concerns that the technology at Greenpeace’s disposal could pose a security risk. “I’m sure any terrorist worth their salt has similar equipment,” he said. “But Greenpeace use things in a very constructive way, otherwise I would not have made the film.”

Kent Police confirmed that it was investigating how the footage could have been intercepted, and said that it would also be investigating other means by which Greenpeace could have been given access to the pictures.

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