European Union leaders have said the Brexit withdrawal agreement is “not open for renegotiation”, despite appeals from Theresa May.
She wanted legal assurances on the Irish backstop to help her deal get through Parliament, after she delayed a Commons vote in anticipation of defeat.
The PM said the deal was “at risk” if MPs’ concerns could not be addressed.
European Commission president Jean-Claude Juncker said there could be clarifications but no renegotiation.
He urged the UK to set out more clearly what it wants, adding that the commission will publish information on 19 December on its preparations for a no-deal Brexit.
Mrs May returned to Brussels for a summit with EU leaders on Thursday, at the end of a week that has seen her first delay the vote on the withdrawal agreement in Parliament, then win a vote of confidence brought by MPs unhappy with it.
She vowed to listen to the concerns of the 37% of Tory MPs who voted against her and was hoping to “assuage” their concerns about the controversial “backstop” plan in the agreement.
Critics say the backstop – aimed at preventing a hard border in Northern Ireland – would keep the UK tied to EU rules indefinitely and curb its ability to strike trade deals.
Conservative MPs demanded changes to the backstop to make it clear that it could not last forever, and the UK could terminate the arrangement on its own.
At the summit, Mrs May was seeking legal assurances that the backstop, if used, would be temporary.
The BBC’s political editor Laura Kuenssberg said one of the ideas being considered in Brussels was whether they could agree a “start date” for a future trade relationship between the UK and the EU after Brexit instead of an end date for the backstop.