More than 50 charities have signed a letter calling on the leaders of the major Westminster parties to commit to repealing the lobbying act because of the "enormous administrative and financial burden" it is placing on voluntary sector organisations.
The letter, which was coordinated by Friends of the Earth, says the lobbying act has "had a significant chilling effect on legitimate charity sector campaigning in the pre-election period" and claims that some charities have had to reduce or alter their campaigning activities because of the act.
The lobbying act sets spending limits and makes it a legal necessity for all organisations that spend more that £20,000 in England or £10,000 in Wales on regulated campaigning prior to an election to register with the Electoral Commission.
The development charity umbrella group Bond said it and several of its members had signed the letter.
"Voices are being lost at this crucial time, and our democracy is the poorer for it," the letter says. "Those charities that continue to campaign to further their charitable objects are subject to an enormous and unreasonable administrative and financial burden.
"This is compounded by the retrospective year-long regulated period, which requires us to go back and assess campaigning from before we knew a general election would be called."
The letter has been sent to the major parties from Great Britain represented in Westminster: the Conservatives, Labour, the Scottish National Party, the Liberal Democrats, the Green Party and Plaid Cymru.
The other three parties have made no explicit promises to repeal the act.
The letter comes after recent fines of £1,000 and £30,000 handed out to Friends of the Earth and Greenpeace respectively for failing to register with the Electoral Commission in relation an anti-fracking campaign the two charities ran before the 2015 general election.
Greenpeace subsequently threatened to take the Electoral Commission to court over its decision to retrospectively apply the election spending rules to the year before the snap election on 8 June.
Craig Bennett, chief executive of Friends of the Earth, said: "The problem with the lobbying act is that it seriously damages charities’ ability to do their job, and that job is to work for the greater public good.
"If the lobbying act is not reformed, democracy will ultimately suffer."
Tamsyn Barton, chief executive of Bond, said: "Campaigning during election time is a critical and legitimate part of what our members do. This is how charities ensure those who are vulnerable and marginalised have a voice. But the level of red tape involved in complying with the act is excessive and the cost of legal advice is unaffordable for smaller organisations, especially at such short notice.
"Shockingly, many organisations have felt unable to speak out during this snap election on issues that affect the people they aim to represent, and our democracy is all the poorer for it."
But several prominent people in the charity sector, including Sir Stuart Etherington, chief executive of the National Council for Voluntary Organisations, have said charities have been "unduly stifling their voices" by misunderstanding the act.
Sir Stephen Bubb, former chief executive of the charity chief executives body Acevo, and the philanthropist and campaigner Gina Miller have also called for charities to speak out more ahead of the general election.