The government has lost its bid to relax rules around the pollution of waterways after a rebellion in the Lords.
Levelling Up Secretary Michael Gove unveiled the plan last month, saying scrapping so-called "nutrient neutrality" measures would free up developers and lead to thousands more homes being built in England.
But the Tory amendment introduced in the Lords - which would have seen the policy tagged on to the Levelling Up and Regeneration Bill - was rejected by peers over the risk it would pose to the environment.
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Nutrient neutrality rules were first introduced across the EU back in 2017, designed to stop developers from polluting local wetlands and waterways in protected areas when building homes.
In practice, it meant companies had to show how they would prevent or offset that pollution in order to win planning permission.
Developers claimed new homes only made a "negligible contribution to the river pollution", so scrapping the measure would help ramp up projects - something Mr Gove and the government agreed with.
But opposition parties and environmental campaigners said it would lead to even more issues in the country's waterways.
Putting the plan to peers, communities minister Baroness Scott said the powers were "necessary and proportionate".
She said the current rules had "effectively stalled or blocked completely housing development in affected areas" and were "burdensome and expensive".
But Labour's Baroness Jones said scrapping the measure would set a "dangerous precedent".
And Tory former environment minister Lord Deben - who chaired the Climate Change Committee until recently - said it was "one of the worst pieces of legislation I have ever seen and I've been around a long time".
After the vote rejecting the policy, shadow levelling up secretary Angela Rayner called the defeat "humiliating" for the government, and said the "flawed plan" was just an attempt to "score cheap political points".
She added: "We stand ready to sit down with the government, housebuilders and environmental groups to agree on a workable solution to build the homes we need.
"If they refuse this opportunity, ministers have only themselves to blame."
The Liberal Democrats' Lords spokesperson, Baroness Pinnock, called the vote "a great victory", adding: "The Conservatives have continually promised not to roll back our environment rules, it is deeply shocking that they can't be trusted to keep their word."
But posting on X - previously known as Twitter - the Conservative Party said: "Starmer and Labour just voted to block 100,000 homes.
"Why? Short-term politics over the needs of British families."
The current government has pledged to build 300,000 new homes every year by the mid-2020s.
Parliamentary figures show housing supply has increased year-on-year from a low point of 125,000 in 2012/13, reaching a high point of 243,000 new homes in 2019/20 - but they have not yet reached the target.