(CNN) UK Prime Minister Theresa May declared Brexit negotiations were "in the endgame" Monday night, but warned that clinching a deal was "immensely difficult."

Addressing leaders of London's financial district, where anxiety is mounting about the economic impact of a no-deal Brexit, May said that an agreement would not come "at any cost."

"Any deal must ensure we take back control of our laws, borders and money," she said. "It must secure the ability to strike new trade deals around the world."

Michel Barnier, the chief European Union negotiator, earlier that day told European ministers negotiations had run into the early hours of Monday morning.

Talks were so intense and protracted that Barnier canceled a planned press conference Monday so he could continue thrashing out a deal, a European diplomat told CNN.

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