UK Peers look set to inflict a second defeat on their government's Brexit Bill by demanding a "meaningful" vote on the final divorce deal.
Independent crossbencher Lord Pannick QC said British Prime Minister Theresa May had accepted the need to give Parliament a vote on the terms negotiated.
But she was refusing to include that promise in the European Union (Notification of Withdrawal) Bill currently going through Parliament, which will allow the Government to trigger the Article 50 process of leaving the EU.
Moving an amendment to the Bill, Mr Pannick, who represented lead claimant Gina Miller in the successful Article 50 legal challenge, said the British parliament should be required to approve the deal at the end of the negotiating process.
But Tory former leader Lord Howard of Lympne said the change would give the Lords a "statutory veto" on the Prime Minister's decision.
And Tory former Cabinet minister Lord Forsyth of Drumlean warned of a "hidden agenda" behind the move to reject the expressed will of the people in the EU referendum.
Earlier the UK Government comfortably defeated a Liberal Democrat bid to give voters a final say on the Brexit deal in a referendum.
The move to put the outcome of Brexit negotiations to a referendum was heavily defeated by 336 votes to 131, Government majority 205.
The legislation, to trigger Article 50 and start the Brexit talks, is set to clear the Lords later in the day but will have to go back to the Commons after last week's heavy defeat over the rights of EU nationals already living in the UK to remain.
In report stage debate on the Bill, Lord Pannick said: "The purpose of this amendment is very simple. It is to ensure that at the end of the negotiating process the approval of Parliament is required for the terms of our withdrawal from the EU."
He said the Prime Minister's commitment to such a vote should be written into the Bill "no ifs and no buts".
The Labour-led amendment, tabled with Lib Dem and crossbench support, also required the approval of both Houses if the Prime Minister decided that the UK should leave the EU without agreement on the terms.
"It must be for Parliament to decide whether to prefer no deal or the deal offered by the EU," Lord Pannick said.
The amendment wouldn't delay notification of withdrawal or commit the Government to adopt any specific approach in the negotiations.
"It doesn't impede the Government in the negotiations any more than the undertaking already given by the Prime Minister.
"It will guarantee that the Government must come back to both Houses and seek approval for the result of negotiations."
Lord Forsyth went on to accuse Lord Pannick of a "clever lawyer's confection".
Responding to an intervention from Lord Pannick, Lord Forsyth said: "This House is absolutely full of people who still haven't come to terms with the results of the referendum, and this is a clever lawyer's confection in order to reverse the results of the referendum."
This prompted groans from other peers, as did further comments that the amendment would tie the Prime Minister down.
Lord Forsyth said: "These amendments are trying to tie down the Prime Minister.
"Tie her down by her hair, by her arms, by her legs, in every conceivable way in order to prevent her getting an agreement, and in order to prevent us leaving the European Union."
Lord Turner of Ecchinswell, a former chairman of the Financial Services Authority, said: "Any idea that the vote of last June reflects the will of the people in some unanimous, all the people together, Una Voce, unanimous, absolute and forever unchanging fashion is ... not democratic, but the Brezhnev Doctrine.
"I'm a bit surprised and depressed by the fact that this Brezhnev Doctrine, having first been propagated by some of our major newspapers, is now finding an echo chamber among some parts, but definitely not all parts, of the Conservative Party."
The Brezhnev Doctrine was used by the Soviet Union as justification to intervene in the affairs of Communist countries to strengthen Communism.