Charities have been forced to change their key messages to the public during the general election because of the “chilling” effect of the controversial Lobbying Act, a group of leading UK organisations has warned.

Democratic debate on some of the biggest issues in the election campaign has been stifled by the law, a group of more than 50 charities writes in a letter sent to the main party leaders.

“Voices are being lost at this crucial time, and our democracy is poorer for it,” they said. Their concerns echo those of many charities, particularly in the field of social care, which told the Guardian they were unable to raise vital concerns over, and experiences of, the impacts of current and future policies.

The Lobbying Act restricts what non-governmental organisations can say in the year before a general election. Billed as a brake on corporate lobbying as well as NGOs when it was brought in, its provisions have fallen harder on the non-profit sector, leading to an independent commission and the House of Lords recommending amendments.

In their letter sent on Monday, more than 50 UK charities called for the urgent reform of the controversial legislation, which they said was having a “chilling effect” on debates over policy ahead of Thursday’s snap election. They warned that charities were “weighed down by an unreasonable and unfair law which restricts our ability to contribute fully to a democratic society”.

The charities come from across the spectrum, representing social care, health, poverty, environment, and vulnerable groups. They include household names such as the hospice and neurological care charity Sue Ryder, AgeUK, Amnesty, the development charity Care, and Christian Aid.

Craig Bennett, chief executive of the Friends of the Earth charity network, which coordinated the letter along with the Bond group of more than 450 development charities, said: “The problem with the Lobbying Act is that it seriously damages charities’ ability to do their job to work for the greater public good. Important civic voices that speak for the most marginalised are being lost. If the act is not reformed, democracy will suffer.”

Tamsyn Barton, chief executive of Bond, added that smaller and poorer charities were among the worst affected as they could not afford legal advice and struggled with the amount of red-tape compliance that the act involves.

Several charities have had to reconsider key publicity and events for fear of falling foul of the act. For instance, Christian Aid’s annual fundraising week fell after the prime minister Theresa May called the snap election. The charity had to deal with thousands of local activists concerned that their leaflets and fundraising events might fall under the act, and spend time and effort logging every activity closely, because the act makes charities liable for their activities retrospectively.

Several charities said that as well as the provisions of the act, they were concerned that some organisations were censoring themselves, thereby damaging democratic debate.

Greenpeace recently became the first charity to be fined for refusing to be regulated under the law, in an act of civil disobedience.

Theresa May’s decision to call a snap general election caused particular consternation because it means all charities’ communications in the preceding year fall under the rules retrospectively. When the act was introduced under the coalition, charities were reassured by ministers that parliamentary elections would be on a five-year cycle, giving them time to formulate and publicise key messages ahead of the formal start of any election campaign.

A review by the Conservative peer Lord Hodgson found that “the right balance” had not been struck in the act “as presently drafted”, and a House of Lords committee found the rules “threaten the vital advocacy role of charities”.

Labour and the Green party have called for the legislation to be repealed, while the Liberal Democrats and the SNP have urged reform. Baroness Parminter, the Lib Dem peer, said the act was “pernicious” and was having a chilling effect on democratic debate.

Within the next few days, the UN special rapporteur will present what is expected to be a critical review of the act and its impact.