Peers have defeated the government on the issue of staying in a UK-EU customs union after Brexit.
Lords voted by 348 to 225 in favour of a plan requiring ministers to report on steps to negotiate a continued union.
Backing it, ex-EU Commissioner Lord Patten said the UK would be worse off unless current arrangements continued.
But Brexit minister Lord Callanan said it required the government to report on the steps taken towards an objective it has “clearly ruled out”.
Number 10 argues that remaining in the customs union would prevent the UK from signing third-party trade agreements with countries across the world.
Lord Callanan signalled the government’s intention to overturn the measures at a later stage, saying before the vote it had no intention to “reflect further” on the matter.
The defeat is the first of several expected as the House of Lords – in which the government does not have a majority – debates the government’s flagship EU Withdrawal Bill in detail.
The bill will end the supremacy of EU law in the UK while ensuring existing laws passed since the UK’s entry in 1973 continue to apply.
A total of 24 Conservatives voted against the government on the customs union amendment, including former ministers Lord Heseltine, Lord Lansley and Lord Willetts.
Serving notice that he would be a serial rebel during the bill’s passage, the former EU commissioner said: “There are times in one’s political career where what is alleged to be party loyalty comes way behind trying to stand up for the national interest.”
However, former Conservative Chancellor Lord Lawson said Lord Patten was putting forward a “political argument dressed up as a trade argument” and it amounted to a “wrecking amendment”.
Remaining in a customs union while leaving the EU would leave the UK in a “quasi-colonial” status, he said.
“I can see there are political reason for remaining in the EU, but I think the political reasons for leaving are much stronger. But what it is absolute nonsense to suggest is that there is an economic case for what is being proposed.”
Lord Forsyth went further, suggesting it was a plot by “Remainers in this House, who are the majority, who refuse to accept the verdict of the British people – and I believe they are playing with fire”.
“I say to colleagues in this House, ‘Have a care to what we are doing.’ We are an unelected house, and this amendment [is] part of a campaign which is putting peers against the people.”
Tory MP and Remain supporter Anna Soubry tweeted that the Lords had “put their country first”, and called on her colleagues in the Commons to back them when the amendment returns.
Labour’s Chuka Umunna also praised the vote, saying it had given the House of Commons a “great opportunity” to discuss the impact of leaving the customs union.
But a statement from the Department for Exiting the EU said the fundamental purpose of the bill was to prepare the UK’s statute book for exit day, not about the terms of departure.
“This amendment does not commit the UK to remaining in a customs union with the EU, it requires us to make a statement in Parliament explaining the steps we’ve taken,” a spokesman said.
“Our policy on this subject is very clear. We are leaving the customs union and will establish a new and ambitious customs arrangement with the EU while forging new trade relationships with our partners around the world.”