EU chief Brexit negotiator Michel Barnier with Irish Taoiseach Leo Varadkar | Clodagh Kilcoyne - WPA Pool/Getty Images Barnier to next British PM: May’s deal ‘only option’ for orderly Brexit Commission’s chief negotiator says nostalgia, City rule-breakers and austerity fueled Leave vote.

If the U.K. wants to leave the EU with a deal, the current Withdrawal Agreement is "the only option," according to the European Commission's chief Brexit negotiator Michel Barnier.

"If the choice is to leave without a deal — fine. If the choice is to stay in the EU — also fine. But if the choice is still to leave the EU in an orderly manner, this treaty is the only option," Barnier told the New York Review of Books in an interview published Wednesday. "This is all that our legal constraints allow."

While numerous Tory leadership contenders say they want to scrap the Irish backstop in the Withdrawal Agreement in favor of so-called alternative arrangements, Barnier had a message for whoever succeeds Theresa May: Sign the treaty, and you can pursue alternative arrangements afterward. Otherwise, it's no deal.

"We said that after the Withdrawal Agreement is accepted, we can immediately start work on a long-term solution for the Irish border," Barnier said. "It could be, for example, what we call alternative arrangements: technology, drones, invisible controls. None of these arrangements are operational today."

He added: "You cannot do it immediately. They [the technologies] must be fully operational. I was the French minister of agriculture — how do you control a single cow or a truckload of pork? You need technology. You need technical infrastructure to do that. And this takes time."

Barnier pointed to contradictions in the Tories' demands of the EU. "You cannot leave the EU, the single market, the customs union, and ask for no checks and controls at the same time," he said. "Three types of controls are needed at every external border of the Union. To protect consumers and the EU’s budget, to protect business and stop counterfeiting. It’s not ideological: What is at stake in Ireland is peace and stability. Protection of the single market is important, but only second to peace and stability."

Brexit should be used as a cautionary lesson for the entire EU, the veteran French statesman said.

"For Britain, it’s probably too late, but it’s not too late for other countries where we have exactly the same problems, including my country," he said. "Looking at the causes of Brexit, we also find typically British reasons: the hope for a return to a powerful global Britain, nostalgia for the past — nostalgia serves no purpose in politics. In my country, too, some politicians still prefer to live in the past."

And he said City of London rule-breakers and austerity also fueled the Brexit vote.

"Some based in the City of London voted to leave, as they don’t want to accept the Union’s regulations on their trading; they want to speculate freely and the Union doesn’t allow them to do so," he said. "And most importantly, there are many people who feel abandoned. They feel that the quality of public services, health care, transport, is worsening. We must listen to these fears and address them."