Green groups are calling on ministers to put their commitment not to weaken environmental standards post-Brexit into law as part of the retained EU law bill.
Peers are tabling motions on Tuesday for an environmental baseline to be put into the controversial bill, which is designed to formalise which EU-derived legislation is brought permanently into UK law.
The majority of environmental laws in England and Wales are derived from the EU, so green groups are concerned that any changes to laws could weaken protections for wildlife, habitats, and water, including laws on air pollution, and rules to make developers offset the impact of increased sewage from new homes.
Ministers have said "environmental protections will never be downgraded" in the bill but refused to accept previous Lords amendments to make this part of the law. Instead the government whipped its MPs to reject the draft of the bill containing legal safeguards to stop any environmental laws being downgraded.
As the bill returns on Tuesday to the Lords, peers are tabling two more amendments to put an environmental baseline into the bill and prevent protections being downgraded.
Ruth Chambers of Greener UK, a coalition of 10 major environmental organisations including the National Trust, RSPB and The Wildlife Trusts, said: "The more that the government refuses to compromise on this amendment, the more suspicious its motives appear. Why would a government that says it is committed to environmental protections continue to resist adding a protective layer in this bill?
"It is an uncontroversial change that would provide reassurance to people worried about air pollution and water quality. We hope ministers will seek sensible compromise."
The government has rowed back on plans to ditch all remaining EU laws by the end of the year. It now aims to remove 800 statutes and regulations, instead of 3,700 laws it had lined up for a "bonfire" of EU law in December.
Last month peers inflicted a major defeat on the government, voting in favour of an amendment calling for greater parliamentary scrutiny of the EU laws to be scrapped at the end of the year and an amendment ensuring that future governments cannot weaken existing levels of environmental protection.
On Tuesday peers will debate a motion by Lord Krebs that any changes to regulations do not reduce the level of environmental protection arising from the EU retained law to which the provision relates.