Leading charities are calling on the next government to lift the legal gag that prevents them from campaigning for the poor and marginalised in society.

Heads of non-governmental organisations, including Friends of the Earth, Shelter, Liberty, End Violence Against Women, the Quakers and the Howard League, have written to all of the main political parties calling for greater freedom to speak out.

The charities say they are being prevented from representing the most vulnerable in society through the restrictive requirements of the Lobbying Act. They are pushing for it to be significantly revised and for the next government to uphold the right to protest and speak out.

The act has been labelled a gagging law for charities; it controls what they can say and do publicly in the 12-month run-up to elections. It aims to ensure that individuals or organisations cannot have an undue influence over the vote.

The act was aimed at controlling the influence of business lobbying and wealthy individuals, but NGOs say it has had a chilling effect on charities in the run-up to elections.

Increasingly, they say, restrictions have been placed on the ability of civil society groups to speak out against policies that could harm vulnerable people in the UK and around the world.

The charities say despite the government’s recent warm words about promoting democracy and civil society voices around the world, in the UK both are in decline. The UK has joined a list of 12 European countries in which civic space is rated as “narrowed”.

The letter, sent to Boris Johnson, Jeremy Corbyn, Jo Swinson, Nicola Sturgeon, Siân Berry and Jonathan Bartley, Nigel Farage and Adam Price, says: “It is essential that this freedom to campaign is restored. The UK can only be a beacon to the rest of the world if it respects civil society’s right (and sometimes duty) to campaign.”

Stephanie Draper, the chief executive of Bond, the international development network organisation, said: “Civil society organisations play a critical role in ensuring the voices of the most vulnerable people are heard by policymakers. However, it is getting harder and harder for these organisations to perform this function which means policies are at times being made with little representation from the people most affected.

“Any future government must take urgent steps to revise the Lobbying Act, ensure that all government departments work with civil society throughout the policymaking process, and remove any restrictions on challenging government policy publicly from grants and contracts.”

Paul Parker, the recording clerk of Quakers in Britain said: “Civil society must continue to speak truth to power. Deterring civil society campaigning is short-sighted, and risks missing insights vital to policy change. At election time it is vital that politicians don’t cover their ears, but listen to charities, faith groups and civil society organisations working with the most vulnerable.”