British Prime Minister Theresa May said Monday that negotiators were working through the night to get a Brexit deal with the EU but vowed it would not be "an agreement at any cost".
"The negotiations for our departure are now in the endgame," May's office quoted her as telling a reception for officials of London's financial district.
"And we are working extremely hard, through the night, to make progress on the remaining issues in the Withdrawal Agreement, which are significant.
"Both sides want to reach an agreement. But what we are negotiating is immensely difficult. I do not shy away from that."
She added: "I will not compromise on what people voted for in the referendum. This will not be an agreement at any cost."
Britain has been pressing hard in recent days for a withdrawal deal with the European Union, hoping the bloc can call a leaders' summit later this month to approve the agreement.
Waiting until the next formal EU summit in mid-December reduces the time May has to get the deal through the divided House of Commons before Brexit day on March 29.
But talks are blocked on the issue of how to keep open the border between Northern Ireland, part of the UK, and Ireland.
Eurosceptics in May's Conservative party, who have long complained about her leadership over Brexit, fear the pressure of time will cause her to give away too much in the negotiations.
"The Brexit talks are not about me or my personal fortunes," the prime minister told the Lord Mayor's Banquet.
"They are about the national interest -- and that means making what I believe to be the right choices, not the easy ones.
"Overwhelmingly, the British people want us to get on with delivering Brexit, and I am determined to deliver for them."
Citing the promises of the 2016 referendum, she said any deal must ensure Britain can "take back control of our laws, borders and money".
It must also allow Britain to strike its own independent trade deals, protect jobs, security and the union of England, Scotland, Wales and Northern Ireland.