Peers backed by a majority of 95 a change to the Agriculture Bill aimed at blocking the import of foodstuffs produced abroad at lower animal welfare standards.
Voting was 307 to 212 for an amendment requiring food products imported under future trade deals to meet or exceed domestic standards, to prevent UK farmers being undermined.
It was the second defeat for ministers in Tuesday's report stage debate on the legislation, as the Lords earlier backed a move demanding a ban on the use of pesticides near homes and public buildings or spaces, such as schools and hospitals.
For the opposition, Lord Grantchester warned: "Low-quality food cannot be allowed to jeopardise rural communities by undercutting UK farmers with products using methods that would be illegal here."
Consumers did not want chlorinated chicken or hormone-treated beef to be imported from the US, he said.
It was vital to signal to existing and future trade partners that the UK was committed to championing high quality standards in food, Lord Grantchester added.
Independent crossbencher Lord Krebs, a former chairman of the Food Standards Agency, said there were "uncertainties" over assurances given by ministers on the issue.
It was only by supporting the Labour-led move that peers could be sure the Government was "bound to its commitment not to import food of lower standards than our own domestic products", Lord Krebs said.
Baroness Boycott, a crossbench peer, said chlorinated chicken was the "tip of the iceberg" of "bad food" which could come into the country.
"Low-quality food is unhealthy food," she said. "It's usually meant deforestation in its production and terrible treatment of animals."
Green Party peer Baroness Jones of Moulsecoomb said the amendment was essential to protect British farming against "vile" American animal welfare standards.
But Tory peer Baroness Noakes said: "The government's policy is clear. They are committed to higher food and welfare standards.
"We do not need to write into law what the Government is committed to."
Conservative former Cabinet minister Lord Lilley urged the House to think twice "before going back more than a century to introduce protectionism, to flouting international law and to doing something whose sole purpose is to raise the cost of food".
Environment minister Lord Gardiner of Kimble said the government would not compromise on its manifesto commitment to the UK's high food standards in trade agreements.
He said none of the 20 continuity agreements signed to date had undermined domestic standards.
The government was dedicated to improving animal welfare standards and had robust processes in place to protect them.
Existing legislation retained standards to maintain the same "high level of protections for both domestic and imported products," he said.
This included a ban on the import of hormone-treated beef, and poultry washed in anything other than potable water, Lord Gardiner said.
The reverses for the government follow two defeats last week on the Bill, which introduces a new support system for farmers as the UK quits the EU-wide Common Agricultural Policy post-Brexit.
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