A wildlife charity said it will fight proposed changes to UK water pollution laws that it claims will hit habitats.
Rishi Sunak, who visited Norwich on Tuesday, confirmed government wanted to enable house-building by seeking to amend rules on "nutrient neutrality".
The EU regulations have meant developers and councils had to ensure new housing did not cause additional pollution to rivers and waterways.
Norfolk Wildlife Trust said it was "flabbergasted" by the proposals.
It added it was a "weakening" of the legislation.
The leader of South Norfolk Council, John Fuller, who has campaigned for the changes, said since the EU law was passed in 2017 some 16,000 homes across Norfolk have been put on hold due to the nutrient neutrality rules.
They affected developments near protected areas such as The Broads and the River Wensum.
Councils faced being prosecuted if housing, given planning consent, saw any wastewater polluting rivers.
Mr Fuller said: "For the first time it places responsibility with those organisations that are accountable for cleaning up our rivers and making sure they do so.
"The Environment Agency, Defra, Natural England and yes, the water companies, are going to be absolutely put on the spot rather than try to palm off the responsibility to councils."
The Wildlife Trusts, as a national body, accused government of "lying" to the public about its commitment to protecting rivers and water courses.
Gareth Dalglish of Norfolk Wildlife Trust said: "I'm flabbergasted the government has announced this after all the furore from the public over concerns about pollution and sewage in rivers."
Mr Daglish said the legislation "protects our rivers and waterways from more pollution, so it doesn't even reduce the existing."
He called the move an "erosion of standards", adding it could have a wider impact on the Broads.
"Not just the wildlife, but the people and Norfolk's [leisure] economy, since no one wants to paddle in blue-green algae, caused by excess phosphates and chemical run-off."
While most waste water is treated at sewage treatment plants, some can occasionally end up in watercourses.
The government wants the rules on nutrient neutrality to now be seen as guidance rather than laws.
Mr Sunak said the changes would be "fantastic news for Norfolk, which has been particularly impacted by this disproportionate old EU rule, that stopped building happening."
The government said water pollution from new homes was "very small" and would be offset by £280m of investment for environmental measures, run by Natural England.
The new environmental measures would mostly target farming and water companies to restore protected waterways.
The PM said the water regulator, OFWAT, had been given more powers and could fine firms larger penalties over water pollution infringements.
The government will endeavour to scrap these rules through an amendment to the Levelling Up and Regeneration Bill, currently going through the House of Lords.
The Wildlife Trusts planned to lobby MP's and other organisations to try and prevent any changes.