Tory grandee Lord Heseltine has vowed to rebel against Theresa May over Brexit and back opposition efforts in the House of Lords to ensure a meaningful vote on the final outcome of exit negotiations.
The former cabinet minister insisted he was not engaging in a "confrontation" with the Government.
He said Labour, Liberal Democrat and rebellious Tory colleagues ready to back a change to the Brexit Bill simply want to uphold the Supreme Court's ruling that MPs and peers have ultimate authority.
A Labour Lords source said the party "would be likely to win handsomely" and inflict defeat on the Government on the issue, as well as in a bid to guarantee rights of EU nationals already in the UK.
A vote on whether Parliament should have a meaningful say on the final deal is not expected during the European Union (Notification Of Withdrawal) Bill's committee stage next week, but is more likely during report stage on March 7, the source said.
Writing in The Mail On Sunday, Lord Heseltine said: "In the end the outcome of Brexit will have to be confirmed by Parliament.
"It will also have to pass in 27 national European parliaments, several sub-national parliaments and the European Parliament.
"It was perhaps unwise for our Government to suppose that our Parliament should be excluded where all others were included.
"Very sensibly, after the Supreme Court interpreted the law, that position was reversed and Parliament was restored to its rightful constitutional role as the ultimate authority.
"I will vote in the House of Lords to ensure that position is legally intact.
"This is not a confrontation with the Government which has already made such a commitment.
"It is, put simply, a decision to ensure that the Commons has the chance to define its role in the exercise of its authority over what most people regard as the defining issue of our time."
Lord Heseltine also suggested the Brexit decision could be reversed if public opinion changes.
"My opponents will argue that the people have spoken, the (Brexit) mandate secured and the future cast," he said.
"My experience stands against this argument.
"At the moment there is no evidence that public opinion has changed. The PM rides high in the polls. But what if this changes?"
If the Bill, which will give the PM the power to invoke Article 50 to begin exit negotiations, is amended, a period of parliamentary "ping pong" may follow, with MPs asked for vote on changes made in the Lords.
But Mrs May expects to be able to stick to her timetable of beginning talks with Brussels by April.
Gina Miller, who alongside other campaigners brought the legal action which led to the historic Supreme Court decision, urged peers to avoid MPs' "cowardice" on the issue.
The House of Commons voted the Bill through unamended and with a large majority of 372.
Ms Miller told the Independent: "I think it's absurd this idea that the Lords should just get on with it and not have a proper debate.
"After we fought so hard to put the Commons back at the centre of the debate, all the Commons did was rubber stamp the bill, which was cowardice.
"I am hoping the Lords actually do what they should be doing constitutionally, exercising their parliamentary sovereignty, being independent, scrutinising the Government and looking to put in amendments."