Government officials have been monitoring environmental campaign groups and then passing intelligence on to the police, according to documents seen by the Guardian.

An internal risk report from the Department for Transport reveals that a unit referred to as the comms directorate ­"continuously monitor[ed]" peaceful protest groups opposed to the expansion of Heathrow airport and then briefed detectives about their findings.

The disclosure is the latest evidence of a wide-ranging crackdown on environmental campaign groups and has been condemned by MPs and civil liberty groups.

Earlier this month it emerged that government officials had handed confidential police intelligence about environmental activists to the energy giant E.ON ahead of a peaceful demonstration at Kingsnorth, the proposed site of a new coal-fired power station in north Kent.

Last week the Guardian revealed how undercover police were running a network of hundreds of informants inside protest organisations who secretly feed them intelligence in return for cash.

The latest documents, released under the Freedom of Information Act, reveal how the comms directorate at the DfT was monitoring campaign groups in the consultation period leading up to the announcement about the planned expansion of Heathrow airport.

A typical entry in February 2008 states: "Strength of opposition to expansion at Heathrow leads to direct action during the consultation period." Under measures in place it states: "Comms directorate to continually monitor protest groups and brief staff/police accordingly."

Another entry in May 2008 states: "Strength of opposition to expansion at Heathrow leads to direct action before and around time of ministerial considerations thereby delaying final policy decisions."

The document rates the "inherent risk" as medium or high and states: "Comms directorate to continuously monitor protest groups and brief staff/police accordingly."

The Liberal Democrat justice spokesman, David Howarth, said it was "extraordinary" that civil servants were monitoring peaceful environmental campaigners and then passing information to police.

"The document confirms the suspicion that the main concern of the government is to undermine the protest movement rather than to engage in a genuine debate with campaigners and local residents.

"The fact that the monitoring was being undertaken by a communications and public relations team, rather than by the police or security service officials, strongly suggests that the government's real concerns about protests are political and nothing to do with national security or public order."

Greenpeace said the documents revealed a systematic attempt by government to stop protesters highlighting the impact Heathrow expansion would have on climate change.

"A pattern appears to be emerging of government departments concerned as much with protesters as with policy," said spokesman Ben Stewart. "The case for Heathrow expansion was riddled with exaggeration and misrepresentation and now we know it was pushed through by the Department for Transport over-reaching its authority to spy on peaceful and legitimate protest groups."

The disclosures have added to the growing controversy over the policing of protests. Last month the Met commissioner, Sir Paul Stephenson, requested an independent review of his force's tactics, amid mounting evidence of violent behaviour by officers at the G20 protests.

Denis O'Connor, the chief inspector of constabulary, who will carry out the review, is now expected to look at other demonstrations, including last year's policing of Kingsnorth.

Last night a spokesman for the DfT said its officials had only gathered information that was in the public domain.

"In light of the high-profile opposition to the proposed expansion, the police were notified of where and when Heathrow public exhibitions would be held and the Department made sure it kept abreast of information published on protest groups' websites and coverage in the media to see if any direct action was planned at these events.

"The information gathered was readily available in the public domain, and simply used to alert staff and the police to any disruption that was anticipated at these public events."

Sabina Frediani, campaigns co-ordinator for Liberty, said it was inappropriate for civil servants to be carrying out any investigation into protest groups .

"We always thought a communications directorate was a press office – now that simple phrase takes on positively Orwellian overtones. First we had police investigating Home Office leaks and now we've got DfT officials investigating protest groups. Haven't we had enough of constitutional cross-dressing?"