New rules have come into force which could dramatically reduce the ability of individuals and non-governmental organisations to bring legal challenges to protect the environment.
The government is scrapping automatic "cost caps" which limit the costs of losing a case in England and Wales.
Opponents claim the changes will make it "impossible" to "hold the government to account".
But the government says people will not be expected to pay above their means.
The caps currently stand at £5,000 for an individual and £10,000 for an organisation.
The normal "loser pays rule" means that successful claimants can claim their legal costs back from the defendant.
But if they lose, they have to pay both their own legal costs, and those of the winning side.
Under the changes, any person or organisation wanting to bring a judicial review in environmental cases will not automatically receive the protection of a '"cost cap" if they lose.
That could mean individuals having to sell a house.
ClientEarth, Friends of the Earth and the RSPB are challenging the rule change in the courts, arguing those bringing such cases would be exposed to huge and uncertain financial risk.
The "cost caps" came in in 2013 in part due to the international Aarhus Convention, which was ratified by the government in 2005.
It requires contracting parties to ensure that legal action to protect the environment is "fair, equitable, timely and not prohibitively expensive".
This recognises that the environment cannot protect itself and that there is a public interest in people and groups bringing legal actions to protect it.
Before the caps, the cost of bringing cases could be huge.
In one, concerning the construction of a funicular railway up Cairngorm Mountain in Scotland, WWF was ordered to pay the government's legal costs of over £200,000 on losing.
In another, local resident Lilian Pallikaropoulos faced a costs bill of just under £90,000 after losing her challenge against the legality of a large cement works near her home in Rugby, Warwickshire.
Under the new rules, the court can look at the financial resources of a claimant and discard the automatic cost cap.
This could involve an assessment of how much their house is worth and whether they should be forced to sell it if they lose.
It is estimated that some 40,000 people in the UK die prematurely each year because of air pollution.
The group ClientEarth has brought successful legal challenges against the government's failure to meet EU targets on air pollution.
Its chief executive James Thornton said: "By removing cost caps and allowing personal finances to be publicly examined, it creates a huge deterrent for those who would use law to defend people's health and the natural world.
"With unlimited legal costs, it will be virtually impossible to bring a public interest case and hold the government to account.
"This is especially true after a hard Brexit - which looks increasingly likely - when the EU won't be able to punish UK law breaking."
Campaigners say the UK's public interest cost rules are already more punitive than the US, China, and any other country in the EU.
They claim environmental public interest cases made up less than 1% of all judicial reviews from 2013 to 2015, and that they achieve twelve times the success rate of other judicial reviews.
A Ministry of Justice spokesman said: "The cost of bringing environmental challenges must not be prohibitively expensive and our changes will ensure that individuals are not expected to pay legal costs above their means. Legal aid remains available for these cases".
But last week, a House of Lords committee concluded that "people with a genuine complaint will be discouraged from pursuing it in the courts".