Theresa May's bid to make her Brexit deal more acceptable to MPs has suffered a blow after EU leaders said it was "not open for renegotiation".
She wanted legal assurances on the Irish backstop and had warned the deal itself was "at risk" over the issue.
But European Commission president Jean-Claude Juncker said there could be clarifications but no renegotiation.
Labour says MPs must vote on the deal next week and it was "unacceptable" for it to be pushed back to January.
"This is becoming a farce," said Shadow Brexit Secretary Sir Keir Starmer.
"The prime minister pulled this important vote last week on the basis that she was going to get meaningful changes to her Brexit deal, She has obviously not."
On Thursday evening, Mr Juncker urged the UK to set out more clearly what it wants, adding that the European Commission will publish information on 19 December on its preparations, should the UK leave the EU without a deal in place.
"Our UK friends need to say what they want, instead of asking us to say what we want, and so we would like within a few weeks our UK friends to set out their expectations for us, because this debate is sometimes nebulous and imprecise and I would like clarifications," he said.
Pooled video footage from the summit on Friday showed Mrs May and Mr Juncker engaged in what appeared to be a tense exchange, following his comments.
The Democratic Unionist Party, on whom Theresa May relies for her Commons majority, said the EU's response was unsurprising and Mrs May must not "roll over as has happened previously".
"The EU are doing what they always do," said the party's leader Arlene Foster. "The key question is whether the prime minister will stand up to them."
But Cabinet minister David Lidington described the meeting as a "welcome first step" in showing that the EU was committed "to negotiate a trade deal with the UK speedily".
Mrs May travelled to Brussels to make a special plea to EU leaders after delaying Tuesday's Commons vote on the deal, in anticipation of a heavy defeat.
She then went on to win a confidence vote brought by her own MPs but vowed to listen to the concerns of the 37% of Tory MPs who voted against her and was hoping to address their concerns about the controversial "backstop" plan in the withdrawal 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 have demanded changes to make it clear that it could not last forever, and the UK could terminate the arrangement on its own.