The government has suffered its 15th defeat in the House of Lords over the EU Withdrawal Bill.
Peers voted by a majority of 50 to say the government should set up a body to maintain EU standards of environmental protection after Brexit.
Lord Krebs, who instigated the move, argued that while EU rules would be carried over into UK law, environmental principles underpinning them would not.
Ministers had promised a consultation on the issue but lost by 294 to 244.
MPs will decide whether to reverse the measure when the bill returns to the House of Commons. Peers are debating the third reading of the Bill, the last chance for them to propose changes to the legislation.
The cross-party amendment backed by peers is designed to ensure EU environmental principles continue to have a basis in domestic law at the end of the post-Brexit transition period in December 2020.
It requires the environment secretary to bring forward proposals for primary legislation to create a duty on public authorities to apply these principles, and to establish an independent public body to ensure compliance.
Lord Krebs, the former chair of the Food Standards Agency, said he was "not satisfied" with the idea of a consultation and wanted guarantees that existing principles will continue to apply and be enforced.
"We have heard many times that the purpose of the Bill is to ensure that everything is the same the day after Brexit as it was the day before," he said.
"Yet for environmental protection things will not be the same. We're talking about the protection of our air quality, our water quality, rivers, oceans, habitats and biodiversity."
Brexit minister Lord Callanan argued the proposed change was "premature" in that it prejudged a period of consultation and would "ultimately be detrimental to the future protection of environmental law".