Charities must fight a Ministry of Justice consultation intended to make it more difficult to use the judicial review process to campaign, umbrella bodies and sector legal experts have said.
And a senior lawyer at the law firm Bates Wells Braithwaite said the proposed changes to the law would make it "harder or impossible" for charities to use the law to campaign against government decisions.
Under the proposals, published this month, the government aims to introduce a more stringent test to decide who can bring a judicial review and to make it more expensive for campaigning groups.
"The government is concerned that too wide an approach is taken to who may bring a claim, allowing judicial reviews to be brought by individuals or groups without a direct and tangible interest in the subject matter to which the claim relates, sometimes for reasons only of publicity or to cause delay," the consultation document says.
It says that the government "seeks views on how the approach to costs for judicial review can be adjusted to encourage claimants and their legal representatives to consider more carefully the merits of bringing a judicial review and the way they handle proceedings".
In an article in the Daily Mail, the justice secretary Chris Grayling said he intended to use the change in the law to cut down on the right of charities to campaign.
"The professional campaigners of Britain are growing in number, taking over charities, dominating BBC programmes and swarming around Westminster," he wrote. "While charities inundate Westminster with campaign material, they also target the legal system as a way of trying to get their policies accepted.
"Britain cannot afford to allow a culture of left-wing-dominated, single-issue activism to hold back our country."
Sir Stephen Bubb, chief executive of Acevo, said the move was a "direct attack on charities’ ability to campaign".
A spokeswoman for the NCVO said it would be working with Bates Wells Braithwaite to "highlight the issue, raise awareness, gather views and encourage organisations to submit evidence to the Ministry of Justice".
Neil Cleeveley, director of policy and research at Navca, said: "We will be looking carefully at the implications of this consultation and we will certainly oppose anything that makes it harder for charities to help individuals challenge patently unfair decisions."
Melanie Carter, head of public and regulatory law at Bates Wells Braithwaite, said that if the government’s proposals were implemented, "then charities and other organisations are likely to find it harder or even impossible to use the law as part of their campaigning".
Carter said the proposals, if implemented, would restrict the ability of charities to challenge unfair decisions on behalf of their beneficiaries.
"To deny this role to the sector is to undermine one of the checks and balances in our society," she said. "Moreover, the line between campaigning and simply holding public authorities to account and requiring them to behave lawfully might become blurred.
"One of the critical roles of the third sector is to step in and mount a legal challenge, essentially on behalf of the most vulnerable in our society – if the sector is stopped from doing so, simply because of the nature of the organisation, these groups of people will find themselves even less protected."
The consultation runs until 1 November.