Until last month the small market town of Langnau in the rolling Swiss hills had two claims to fame; it was both a centre for the production of Emmental cheese and also one of the sunniest places in Switzerland. Today, thanks to a routine police traffic inquiry, it has the dubious honour of being the location where one of Europe's biggest alleged acts of eco-terrorism was foiled.

On the night of 15 April local officers pulled over a car on one of the town's quiet streets. Inside the vehicle they found a large cache of explosives, primed and ready to detonate. The three people in the car are alleged to have been members of the murky Italian anarchist group Il Silvestre, who were reportedly on a mission to blow up the nearby unfinished £55m IBM nanotechnology facility.

The apparent attack is believed to be part of a new co-ordinated wave of eco-terror on the continent. The IBM site is due to be opened next year and will be the most advanced centre for nano- and biological scientific research in Europe. According to reports, the eco anarchists Il Silvestre are opposed to all forms of nanotechnology. The group was formed in Tuscany and is considered by some to be one of the rising "eco-terror" groups in Europe, with a rigid cell structure, access to explosives, and a membership that supposedly has no qualms about killing to achieve its goals.

Supporters, on the other hand, argue that the group, which publishes the militant magazine Terra Selvaggia, are "radical ecologists" and "revolutionaries".

The idea that green activists are willing to destroy, maim and kill in their crusade to protect the planet goes against the domestically fostered image of cuddly, eccentric green campaigners epitomised by Swampy, the dreadlocked former public schoolboy sitting in a muddy hole in Devon waiting for the bulldozers to arrive. With labels like tree-hugger, hippy and bunny lover, there is a quaint Britishness about the subterranean Twyford Downs protesters and the Canbury Gardens activists, who lived in tree houses for weeks to save a row of Poplars in Kingston-upon-Thames from the developer's chainsaw in the late Nineties. So when did the cosy eco-warrior become seen as a hardcore terrorist? And just how much of a threat is environmental extremism?

The term eco-terrorist was first coined in 1990 to describe anti-logging activists in the Pacific US who employed the potentially deadly technique of "spiking". Carried out initially by the radical group Earth First, spiking involved driving iron nails into trees to sabotage attempts to cut them down. The Republican senator Denny Smith described the Earth First activists as eco-terrorists when he lobbied the US government for tighter legislation to prosecute them.

Before this date, though, there were individuals who identified with many of the tenets of what we would now consider to be eco-terrorism. The most famous – or infamous – of these is Ted Kaczynski, also known as the Unabomber, who between 1978 and 1995 sent 16 bombs to targets that included universities and airlines, killing three people. "What first motivated me wasn't anything I read. I just got mad seeing the machines ripping up the woods and so forth," he explained in an interview from prison in 1999. "The honest truth is that I am not really politically oriented. I would really rather have just been living out in the woods. If nobody had started cutting roads through there and cutting the trees down I would still just be living there and the rest of the world could just take care of itself."

In 1992 a group of frustrated Earth First activists decided to adopt more radical measures and formed the Earth Liberation Front. The ELF evolved into an international organisation of disparate cells employing confrontational tactics such as arson and intimidation. Along with its sister organisation, the UK-originated Animal Liberation Front, the ELF was hailed in 2002 by James F Jarboe, then-domestic terrorism section chief of the counterterrorism division of the FBI, as the "most active extremist element in the US".

Even after the Twin Towers fell, environmental extremism was seen as a severe threat, and, in 2006, Congress passed legislation – the Animal Enterprise Terrorism Act – which classified certain acts of civil disobedience such as blockades, trespassing, property damage and the freeing of animals, as acts of terrorism.

A recent FBI assessment continued to reinforce fear of environmental radicals when it stated "together eco-terrorists and animal rights extremists are one of the most serious domestic terrorist threats in the US". It highlighted 2,000 crimes committed since 1979 and warned that tactics are "becoming increasingly violent, with threats to life, not just to property".

Acts of eco-terrorism have been directed at housing developments, nuclear power infrastructure, science laboratories and tourist developments. In 2008 the ELF claimed responsibility for burning down a Seattle housing development and also caused $12m (£8m) of damage in 1998 in fire bomb attacks on a Colorado ski resort.

So while eco-terrorists are acknowledged as a threat in post 9/11 America, how much of a danger are they in the UK? Not much, according to the Ministry of Justice, which states that the term eco-terrorist is not used by the British law enforcement community. "It's a label we don't recognise. It's criminal damage, not terrorism," a spokeswoman explains.

However, the National Extremism Tactical Co-ordination Unit (NETCU), a national policing unit, has invented the classification of "domestic extremism" as a label for radical environmental activism – a sort of terrorism-lite.

NETCU states: "There are a small number of single-issue groups and individuals who have pursued a determined course of criminal activity in recent years in the name of issues ranging from animal rights to GM crop studies."

NETCU guidelines state that these extremists are those "whose activities go outside the normal democratic process and engage in crime and disorder". It lists actions as public disorder, malicious communication, blackmail, product contamination and the use of improvised explosive devices but admits that domestic extremist campaigns rarely endanger life.

Despite this, the Ministry of Justice, which does not acknowledge eco-terrorism, has linked environmental extremists alongside far-right activists, dissident Irish republicans, loyalist paramilitaries and al-Qai'da-inspired fundamentalists in guidelines to the probation service. A document from the ministry's National Offender Management Service defines domestic extremism as any "unlawful action that is part of a protest or campaign" and states that "the UK faces a continuing threat from extremists who believe they can advance their aims by committing acts of terrorism".

David Howarth, a former Liberal Democrat MP and law professor, calls the comparison "an astonishing conflation of legitimate protest with terrorism".

In the UK, the ALF has traditionally been seen as the most radical eco-group (its website perpetuates this image with photos of members posing in ski-masks and military garb). It evolved from the hunt saboteur movement and has been operating since the mid Seventies. Comprising a network of individuals and cells it has become more of an ideology than a hierarchical organisation. Robin Webb, who runs the ALF press office, has said: "That is why the ALF cannot be smashed; it cannot be effectively infiltrated; it cannot be stopped. You, each and every one of you: you are the ALF"

Activists say the movement is non-violent even though some have been implicated in acts of intimidation. A sustained campaign against the research facility Huntingdon Life Sciences has included physical assaults on staff, and in 2006 three animal rights extremists involved in the theft of the body of an elderly woman from her grave were jailed for 12 years. The offenders included a vicar's son and a psychiatric nurse; they conducted a six-year campaign against Darley Oaks farm in Newchurch, Staffordshire, which bred guinea pigs for research. Explosive devices were sent to some people linked to the farm, and, in 2004, activists exhumed the body of Gladys Hammond, a relative of the family which ran the farm. The body was recovered two years later in woodland.

While acts like these are abhorrent to most of society, do more common environmental protest crimes such as trespass and vandalism really constitute terrorism? Activists are concerned that simply debating the term eco-terrorism gives it legitimacy. One veteran activist explains that "as soon as we allow ourselves to be labelled eco-terrorists, the authorities can clamp down on us and defend themselves by claiming they are averting terrorist activity".