They are among charities which fear the Coalition Government bill on the issue being debated in the Commons tomorrow is so complex and unclear that it is likely to be "impossible" to follow.
Helen Mountfield QC warned charities will be in "fear of criminal penalty" after she was brought in to assess the impact of the reforms by the National Council for Voluntary Organisations (NCVO), which represents the charity and voluntary sector and has more than 10,000 members.
"Uncertainty about what the law requires is likely to have a chilling effect on freedom of expression, putting small organisations and their trustees and directors in fear of criminal penalty if they speak out on matters of public interest and concern," she wrote in a legal opinion that has been submitted to the Cabinet Office.
"The restrictions and restraints are so wide and so burdensome as arguably to amount to a disproportionate restraint on freedom of expression."
The Transparency of Lobbying, non-Party Campaigning, and Trade Union Administration Bill follows allegations about the influence of lobbyists on Government decision-making as well as the involvement of peers and MPs with lobbying groups.
The Government wants a statutory register of lobbyists, and a £390,000 cap on the amount any organisation - excluding political parties - can spend campaigning in the UK during elections.