The European Union's chief Brexit negotiator Michel Barnier on Monday warned Britain that its departure from the bloc later this week would have "negative consequences" and only ever be an exercise in "damage limitation".
In a speech at Queen's University Belfast, the Brussels diplomat charged with agreeing an EU-UK free trade deal, said nobody had yet convinced him of the "added value of Brexit".
"It is absolutely clear that there will be negative consequences," Barnier said on a visit to Northern Ireland after meeting members of the devolved assembly in the city.
"Whatever agreement we reach on our future relationship, Brexit will always be a matter of damage limitation." Britain will become the first country to leave the bloc at 2300 GMT on Friday, ending nearly five decades of membership.
Barnier had warned earlier Monday during a visit to Dublin for talks with Irish Prime Minister Leo Varadkar that London would have to "rebuild everything" in its new relationship with the EU.
Picking apart some of the pro-Brexit camp's main arguments for its departure, Barnier said the bloc allowed members states "to speak with other superpowers as equals".
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"Brexit is meant to be about 'Global Britain'," he said, referring to a frequently used UK government phrase.
"But, for us, frankly speaking, it is the EU that helps make member states more global," he said in a clear warning to other countries eyeing Brexit closely for their own futures.
"There is no way that any of our countries, alone, could have the same impact."
"I understand the fears of negative economic fallout expressed by some about these checks. But Brexit unfortunately has consequences that we must manage."