The Government has suffered a heavy defeat in the Lords over giving MPs a "meaningful vote" on the final Brexit deal.
Voting was 354 to 235, majority 119, for an amendment to the European Union (Withdrawal) Bill tabled by Tory former minister Viscount Hailsham.
It was worded along the lines of the agreement former Tory attorney general Dominic Grieve thought he had reached with the Government last week.
The vote kicks the controversial issue back to the Commons in a further round of parliamentary "ping pong", with MPs expected to debate it on Wednesday.
Lord Hailsham said the Government had failed to deliver its promise to provide a "meaningful vote" on the Brexit deal and it was up to peers to insist on it to give MPs another chance to debate the issue.
But Lords' leader Baroness Evans of Bowes Park said the amendment had "flaws" and insisted the Government had come back with a "fair, practical and constitutionally sound offer" to meet the demands of peers and MPs.
The vote, at the end of a bad-tempered debate in the Lords, came after the Prime Minister warned that Parliament must not be able to "overturn the will of the British people".
Mr Grieve, who watched the debate from the bar of the House, had warned that the Tory rebels he leads could "collapse" the Government if they disagree with the final outcome of withdrawal talks, and insisted they had the right to a proper say on the Brexit deal.
Lord Hailsham told the Lords: "This Government has sought to prevent a meaningful vote in every possible way.
"I want to ensure that Parliament does have a meaningful vote and I don't want to see that left to chance."
The amendment, which he described as "Grieve 2" and said was in the national interest, required the Government, in the absence of any political agreement by the end of January 21 next year, to make a statement setting out how it proposed to proceed and allow MPs to vote on it.
Tory Lord True said the amendment would give the House of Commons "potential control over the Government in negotiations", adding: "I believe that to be utterly impractical and totally unconstitutional."