- Video report by ITV News Political Editor Robert Peston
The Government has suffered its second defeat on its Brexit Bill as the Lords backed a move to provide a "meaningful" vote on the final deal.
Peers approved an amendment to the Bill by 366 to 268, which would give Parliament a crucial say on Britain's final deal - negotiated by Theresa May - with the European Union.
The amendment to the Bill was supported with Labour, Liberal Democrat and some Tory backing.
ITV News Political Editor Robert Peston said the vote "directly challenged the referendum result".
He added that, if also passed by the House of Commons, the amendment could be used by Parliament to rule that Britain would not leave the EU.
Brexit Secretary David Davis was quick to describe the vote as "disappointing", adding that the Government intended to overturn the result.
- What is a "meaningful" vote?
A "meaningful" vote would give the House of Commons and Lords the opportunity to accept or reject - in two separate votes - the final deal Theresa May negotiates with the EU prior to the UK's departure from the bloc.
In theory, Parliament could delay or prevent Britain's exit by continuing to reject Mrs May's deal.
During the three-hour debate in a crowded House of Lords on Tuesday, peers engaged in a number of bad-tempered exchanges.
Lord Pannick, who represented lead claimant Gina Miller in the successful Article 50 legal challenge, 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."
Tory grandee Lord Heseltine, who also backed the amendment, said the UK was facing "the most momentous peacetime decision of our time" and demanded a "meaningful" vote on the final Brexit deal.
The Conservative former cabinet minister said it would ensure Parliament had a "critical role in determining the future that we will bequeath to generations of young people".
But Tory former leader Lord Howard argued the change would give the Lords a "statutory veto" on the Prime Minister's decision.
And Tory former Cabinet minister Lord Forsyth warned of a "hidden agenda" behind the move by rejecting the will of the people.
After the result of the vote, Mr Davis accused peers of trying to "frustrate" Britain's departure from the EU.
He said: "It is disappointing that the House of Lords has chosen to make further changes to a Bill that the Commons passed without amendment.
"It has a straightforward purpose - to enact the referendum result and allow the Government to get on with negotiating a new partnership with the EU.
"It is clear that some in the Lords would seek to frustrate that process, and it is the Government's intention to ensure that does not happen. We will now aim to overturn these amendments in the House of Commons."
The amendment will now have to be debated and voted on in the Commons.
Previously the Government suffered a heavy defeat over the rights of EU nationals living in the UK to remain post-Brexit - it's first defeat on the Bill.