Peers have defeated the government in voting to give Parliament a potentially decisive say over Brexit.
An amendment to the EU Withdrawal Bill giving MPs the power to stop the UK from leaving without a deal, or to make Theresa May return to negotiations, was approved by 335 votes to 244.
Its supporters said Parliament, not ministers, must "determine the future of the country".
The government will now try to persuade MPs to strike out the change.
Ministers said giving Parliament a decisive say on the Brexit deal risked "weakening" the UK's hand in negotiations.
But Labour said the vote marked a "hugely significant moment" in the fight to ensure Parliament has a "proper role" in the Brexit negotiations and a no-deal situation was avoided.
It was one of three government defeats on the Brexit bill on Monday evening.
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