Prime Minister Theresa May has suffered another Commons defeat after MPs voted down her approach to Brexit talks.
MPs voted by 303 to 258 - a majority of 45 - against a motion endorsing the government's negotiating strategy.
The defeat has no legal force and Downing Street said it would not change the PM's approach to talks with the EU.
But Labour leader Jeremy Corbyn urged Mrs May to "admit her Brexit strategy has failed" and to come forward with a plan Parliament would support.
The defeat came after the pro-Brexit European Research Group (ERG) of Conservative MPs announced it had taken a "collective decision" to abstain, because backing the motion would have amounted to an endorsement of efforts to rule out a no-deal Brexit.
Mrs May has consistently rejected calls to rule out a no-deal Brexit, but Tory Brexiteer rebels believed the wording of what was meant to be a neutral government motion opened the door to that.
The motion reiterated support for the approach to Brexit backed by MPs in votes last month, one of which ruled out a no-deal Brexit.
The voting figures showed it was not just hardline Brexiteers that failed to support the government - a number of Tory Remainers also declined to vote, as more than a fifth of the party in the Commons failed to back the government.
Five Conservative MPs - Brexiteers Peter Bone, Sir Christopher Chope, Philip Hollobone, and Anne Marie Morris, and the pro-Remain Sarah Wollaston - even voted with Labour against the motion.
Downing Street blamed Mr Corbyn for the defeat, saying he had "yet again put partisan considerations ahead of the national interest" by voting against the government's motion.
A No 10 spokesman said the PM would continue to seek legally-binding changes to the controversial Irish backstop, as MPs had instructed her to do in a Commons vote on 29 January.
"While we didn't secure the support of the Commons this evening, the prime minister continues to believe, and the debate itself indicated, that far from objecting to securing changes to the backstop that will allow us to leave with a deal, there was a concern from some Conservative colleagues about taking no deal off the table at this stage," he added.
Plasters lose their stick, revealing the hurt underneath. And the fragile patch that was covering the Tory truce has been well and truly torn.
Just when Theresa May wanted to show the European Union that she could hold her party together to win, she lost.
And at home the prime minister has been shown in no uncertain terms that she simply can't count on the factions in her party to come through for her.
Downing Street had earlier warned that defeat could damage the prime minister's negotiating position, as she seeks to make changes to the controversial backstop "insurance policy" in her deal to avoid customs checks between Northern Ireland and the Republic of Ireland.
ERG deputy chairman Steve Baker told BBC News the group still supported efforts to get "alternative arrangements" to replace the controversial Irish backstop plan, describing Mrs May's defeat as a "storm in a teacup".
But business minister Richard Harrington said ERG members should join former UKIP leader Nigel Farage's new Brexit party, telling them: "In my view you're not Conservatives."
In an interview with The House magazine, he urged ministers opposed to a hard Brexit not to "give in" to the ERG by resigning.
He also said he was "disappointed" that Mrs May had not made a statement to the Commons today, and given MPs an outline of a revised deal to vote on.
"We're now told it will be in another two weeks' time so, being very conscious of the damage that not ruling out a hard Brexit is having on business and industry, I'm concerned that it's going to drag on.
"What concerns me most is there is now talk that there won't be a final decision until the next EU Council on 21 March which, as far as business is concerned, is completely unacceptable."
Going nowhere fast?
By BBC Europe editor Katya Adler
EU leaders still believe this is not the time to budge.
They see the UK arguing, debating and negotiating with itself again - as it has done so often during the Brexit process - rather than engaging with Brussels.
As a result of all this, the new round of EU-UK negotiations are going nowhere fast.
"Window-dressing" is how one senior EU figure described the talks to me - with each side simply repeating their red lines to the other.
So, the current favourite prediction in Brussels is that things will only be resolved in March.
Commenting on Mrs May's latest defeat, Jeremy Corbyn said: "Two weeks ago, the prime minister told Parliament that her new approach could 'secure a substantial and sustainable majority' in Parliament.
"However, tonight's vote has proved that there is no majority for the prime minister's course of action.
"This can't go on. The government can't keep ignoring Parliament or ploughing on towards 29 March without a coherent plan."
He added that the PM needed to admit her strategy had failed "and come back with a proposal that can truly command majority support in Parliament".
Pro-EU Conservative MP Anna Soubry said: "The prime minister has been dealt yet another body blow. This is really serious stuff.
"What is happening is a profound lack of leadership from the very top of government."
She said it was "chilling" that ministers were still keeping no-deal on the table when they had seen economic analysis showing that it would be "absolutely disastrous" for the country.
"What an absolute fiasco this is," she added, blaming a "lack of leadership in both of our broken parties".
Mrs May has promised MPs a final, decisive vote on her Brexit deal with the EU when she has secured the changes to it that she believes MPs want to see.
She believes she can secure a Commons majority for the deal if she can get legally binding changes to the backstop clause - something the EU has consistently ruled out.
A Labour amendment calling for the final, meaningful vote to be held before 27 February was earlier defeated by 16 votes.
An SNP amendment, backed by the Liberal Democrats and calling for Britain's departure from the EU on 29 March to be delayed by three months, was defeated by 93 votes to 315 after most Labour MPs abstained.
Anna Soubry withdrew an amendment calling on the government to publish the latest cabinet briefing on the economic impact of a no-deal Brexit after ministers agreed to meet her and publish relevant documents. Ms Soubry said she would table it again on 27 February if ministers did not keep to their promise.
Brexit Secretary Steve Barclay had pledged to call the EU's chief negotiator Michel Barnier after the vote to discuss the result. The two men are set to resume talks in Brussels early next week.