Government officials have labelled environmental campaigners extremists and listed them alongside dissident Irish republican groups and terrorists inspired by al-Qaida in internal documents seen by the Guardian.

The guidance on extremism, produced by the Ministry of Justice, says: "The United Kingdom like many other countries faces a continuing threat from extremists who believe they can advance their aims by committing acts of terrorism."

It was sent to probation staff who were writing court reports or supervising a range of activists, including environmental protesters.

The advice lists "environmental extremists" alongside far-right activists, dissident Irish republicans, loyalist paramilitaries and al-Qaida-inspired extremists as among groups "currently categorised as extremist [that] may include those who have committed serious crime in pursuit of an ideology or cause".

David Howarth, the Liberal Democrats' justice spokesman, said tonight that the documents revealed "a quite astonishing conflation of legitimate protest with terrorism".

The government has been criticised for tarring environmental protesters as "domestic extremists", a term invented by the police, who say it can cover activists suspected of minor public order offences such as peaceful direct action and civil disobedience. The internal guidance from the Ministry of Justice's National Offender Management Service defines domestic extremism as any "unlawful action that is part of a protest or campaign".

"It is often associated with a 'single issue' protest such as animal rights, far-right and far-left political extremism, anti- war and environmentalist extremism. The activity of Domestic Extremist Offenders is more criminal in its nature than that of an activist – but falls short of terrorism."

"Environmental extremists" are described as committing "criminal activity motivated by the broad philosophy and social movement centred on a concern for conservation and improvement of the natural environment".

Environmentalists reject the extremist label, saying that their protests are peaceful and non-violent.

Harry Fletcher, assistant general secretary of the probation service union, Napo, said its members were unhappy with government attempts to lump environmental campaigners with terrorist suspects.

He said the guidance appeared to be part of a wider attempt by some government officials to interfere for political reasons in the work of criminal justice staff.

Ben Stewart, of Greenpeace, said: "The climate movement has never once sought to further its political aims by using violence, which is something that Jack Straw, foreign secretary during the invasion of Iraq, can most certainly not claim.

"His Ministry of Justice would be better occupied reminding itself that peaceful direct action has a long and noble history in this country."

A ministry spokesman said: "It is not true to say that offenders who have committed criminal offences in connection with an extremist cause are 'all treated the same'. The National Offender Management Service has a programme of work that covers all forms of extremism."