The EU will "never, never, never" compromise on the integrity of the bloc's single market when it comes to negotiating a trade deal with the U.K., according to the EU's chief Brexit negotiator Michel Barnier.

"There will be no compromise on the single market. Never, never, never," Barnier said at Queen's University Belfast Monday evening. "It is our main asset on the EU side," he said. "Never will the EU compromise, (make) fragile or unravel the single market, never," he said.

The EU's single market seeks to guarantee the free movement of goods, capital, services, and labor — the "four freedoms" — within the EU. With a collective population of over 500 million people and consumers, the value of single market membership and that unfettered movement of goods and services is a boon to businesses in the bloc.

When the U.K. leaves the EU on Friday January 31, it will remain a member of the single market but only during a "transition period" until the end of 2020.

During that time, the U.K. and EU will try to strike a trade deal although the short time frame is seen as ambitious and Brussels has warned London that the trading relationship will not be the same post-Brexit.

Irish Prime Minister Leo Varadkar suggested in an interview with the BBC Monday that the EU will be the "stronger team" in post-Brexit trade talks and that striking a deal would be "difficult." U.K. Prime Minister Boris Johnson is bullish, however, saying the U.K. can "wrap up" a deal by its self-imposed deadline of the end of 2020.