Sir Stephen Bubb, chief executive of Acevo, has urged the three main political parties to repeal the parts of the lobbying act that "exert a sinister influence on the charity sector’s voice".
The head of the charity chief executives body has written to David Cameron, Nick Clegg and Ed Miliband, calling on them to repeal Part 2 of the act, which contains the sections that charities are most concerned about, as part of their manifestos for the 2015 general election.
The Transparency of Lobbying, Non-party Campaigning and Trade Union Administration Act 2014 gained Royal assent last month, despite voluntary sector concerns that the legislation would significantly affect their ability to speak out on issues that could be regarded as party political in the run-up to elections.
In a statement, Bubb said the act "commits a grave injustice against civil society, and has seriously damaged trust in politics".
He said the act was "an injustice perpetrated by government", and urged the three main parties to rectify it and to "stand up for community action, liberty and democracy".
"Stand up for our country’s great charitable and campaigning organisations, and ensure that the unfair and unworkable provisions of this act are repealed at the earliest opportunity," he said.
"The new legislation will exert a sinister influence on the charity sector's voice, limiting it in its duty to speak out on behalf of our society’s most vulnerable people, and further damaging the relationship between government and civil society."
The letter says charities must be open and transparent, but "we will not and should not accept that the government has the right to muzzle charities in this way and chill democracy".
Aceco helped set up the Commission on Civil Society, a coalition of more than 130 charities and campaigning groups set up in response to the legislation. The commission is also calling for the parts of the act that affect campaigning by charities to be repealed.