Labour is preparing to vote against the government's plans to scrap water pollution rules to encourage housebuilding.
The House of Lords is set to vote on removing the EU-era "nutrient neutrality" rules on Wednesday evening.
Ministers believe that up to 100,000 new homes could be built by 2030 if the rules are changed.
But the proposal has been condemned by environmental groups, who say it will mean worse water pollution.
Writing in the Times, Labour deputy leader Angela Rayner said her party would table its own proposals through an amendment to the Levelling Up Bill.
If it is not approved, Labour will vote against the government's own amendment on the issue, she wrote, potentially putting the policy at risk of defeat.
Natural England rules currently mean 62 local authorities cannot allow new developments unless builders can prove their projects are "nutrient neutral" in protected areas.
The government has announced plans to scrap these rules through an amendment, or change, to the Levelling Up and Regeneration Bill, currently going through the House of Lords.
It is one of the first major decisions made by Ms Rayner in her new role as shadow levelling up secretary - a role which also has responsibility for housing.
There had been some reports that under her predecessor, Lisa Nandy, Labour was considering backing the government on the issue although these were never officially confirmed.
In the Times piece, Ms Rayner and shadow environment secretary Steve Reed said "there are far better ways to build the new homes we desperately need than green-lighting water pollution".
They suggested developers could be allowed to start building homes before nutrient neutrality plans had been put in place. But the developers would then be required to introduce measures to counteract any environmental harm before the homes were occupied.
This would allow developers to start building homes currently "stuck in the planning pipeline" while allowing extra time for new ways to reduce pollution to become available, they said.
Levelling Up, Housing and Communities Secretary Michael Gove said: "Today, Labour claimed to be the party of homeownership - yet tomorrow, they plan to vote against new laws that would unlock 100,000 new homes and enhance the environment.
"Sir Keir Starmer is attempting to end the dream of home ownership for thousands of families and young people with his political game-playing. Labour are the party of the blockers, not the builders."