And the report - by an advisory group chaired by Labour peer Baroness Kennedy QC - said pressure groups like Amnesty International and Greenpeace should be allowed to run campaigning adverts on TV and radio.

The Advisory Group on Campaigning and the Voluntary Sector also called on Parliament to amend legislation designed to tackle terrorism and stalking, which they said has had the inadvertent effect of stifling legitimate campaigning by charities.

They urged MPs to back a Liberal Democrat bill which would get rid of "disproportionate" restrictions on demonstrating outside Parliament without police permission, introduced in the Serious Organised Crime and Police Act of 2005.

Laws designed to clamp down on harassment by stalkers have been turned against a village community group campaigning to save a local lake, while anti-terrorism measures introduced in 2000 have largely been used by police to stop and search protesters on their way to demonstrations, the report noted.

The legal restrictions have not only seen high-profile prosecutions - such as the case of anti-war protesters fined for reading out the names of Iraq war dead at the Cenotaph - but have had a "chilling" effect on protest groups, who fear they may be charged with harassment if they embark on an e-mail campaign or demonstration.

While recent reform of charity law has given charities some leeway to get involved in campaigning on "political" issues such as human rights and the prevention of poverty, the report said they were still inhibited by a legal framework dating back to 1917 which draws a strict dividing line between charitable and political bodies.

It called for charities to be permitted in future to engage in political campaigning in furtherance of their charitable purposes, so long as they do not support political parties. And it said that the ban on non-charitable "social advocacy" campaign groups like Amnesty and Greenpeace advertising on TV and radio amounted to "unreasonable stifling" of debate.

While major companies like BP and Tesco are able to use expensive broadcast ads to promote their "socially responsible" activities, campaigners are barred from using their own ads to question their claims.

:: The Advisory Group on Campaigning and the Voluntary Sector was set up in February this year and includes representatives of a wide range of charities and pressure groups including Friends of the Earth, Liberty, Oxfam and the RSPCA as well as legal experts and voluntary sector organisations.