In the final years before the Berlin Wall comes down, MI5 and Special Branch still believe parliamentary democracy in the UK is under real threat.

The Secret State is pre-occupied with CND, seen as Moscow's puppets. Madeline Haigh, who once taught Mrs Thatcher's daughter at a private school, feared for the future of her children and joined a local peace group.

She was horrified when she found out that she was being investigated by two undercover Special Branch officers who come knocking at her door.

In True Spies: It Could Happen to You, for the first time Secret State spies from former MI5 chief Stella Rimington to local Special Branch officers, reveal just how wide they cast their net for targets.

Reporter Peter Taylor asks a former Chief Constable "if I had gone on a CND demonstration in the early 80s I was therefore a legitimate person for investigation?", he is told: "Unfortunately yes, you probably could have been, yes."

Although the nature of the threat changes after the end of the Cold War the Secret State still has its hands full, countering animal rights activists, anti-road campaigners and most recently anti-globalisation protestors.

But if the threats are new, the methods used to combat them are time honoured. Special branch officers describe how they manage to defeat the Newbury bypass protestors by planting a spy in their midst and how they turn the full gaze of the Secret State on the leading animal rights activists Robin Webb and Barry Horne, with mixed success.

Reporter: Peter Taylor
Associate Producer: Julia Hannis
Series Producer: Sam Collyns

Please note: Correction

True Spies: It Could Happen To You featured undercover footage showing the mistreatment of dogs in the Huntingdon Life Science labs, first broadcast in Channel 4's A Dog's Life.

A line of commentary mistakenly stated this had been secretly filmed by an animal rights protestor when in fact it was shot by the film's reporter.