The UK and EU are holding talks in a continued effort to reach a post-Brexit trade deal.
Disputes over fishing and business rules remain, with the UK government saying discussions in Brussels have reached "a critical moment".
The EU mood was described as "gloomy" as chief negotiator Michel Barnier met UK counterpart Lord Frost.
Prime Minister Boris Johnson will speak to European Commission President Ursula von der Leyen at 16:00 GMT.
If no deal is done and ratified by 31 December, the UK and EU could introduce import charges on each other's goods.
Speaking in the House of Commons, minister Penny Mordaunt said: "We are at a critical moment in the negotiations."
She added: "We are all working to get a deal, but the only way that's possible is if it's compatible with our sovereignty and takes back control of our country's trade and waters."
The UK was prepared to walk away from negotiations if the EU could not "find compromises", Ms Mordaunt said.
Asked when the cut-off point for a deal was, a European Commission spokesman said: "We are not going to speculate on a last-chance date.
"We are fully committed to substantial negotiations. We've always said and continue to say it's the substance that prevails over timing."
Earlier, Irish Foreign Minister Simon Coveney described the EU negotiating team's frame of mind as "gloomy" and "downbeat".
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