The EU's chief Brexit negotiator, Michel Barnier | Emmanuel Dunand/AFP via Getty Images Barnier tells UK frictionless trade ‘not possible’ post Brexit EU’s chief Brexit negotiator warns of painful economic consequences for Britain.

Listen up, London: Brexit is going to hurt.

The EU's chief Brexit negotiator, Michel Barnier, issued a stark warning to the U.K. Thursday on the painful economic consequences of leaving the bloc, bluntly stating that even under the best circumstances a future relationship providing "frictionless trade" is not possible.

Barnier cautioned that the consequences of failing to reach an agreement on an orderly withdrawal would be even more painful: restoring stiff customs duties, and imposing expensive new costs and "very cumbersome controls and procedures" on British firms wanting to do business in the EU.

Barnier's warnings, in a speech to the European Economic and Social Committee, an advisory body of employers, unions and other interest groups, come 10 days before the next round of bargaining with the U.K. and his remarks reflect a clear concern about unrealistic expectations among his counterparts across the negotiating table.

Thursday's intervention is not the first time Barnier has criticized the U.K. for downplaying the consequences of Brexit. In May when he presented the European Commission’s proposed negotiating directives, which will frame his goals in the divorce talks, the EU negotiator said some had "created the illusion" that Brexit will be quick and painless. This would not be the case, he said. On Sunday Barnier dismissed a suggestion from U.K. Environment Minister Michael Gove that Britain will leave the London Convention on fishing rights when it leaves the EU. "UK denunciation of London Convention=no change," Barnier tweeted.

The EU negotiator's speech Thursday also reflects a new willingness to look ahead to the "future relationship" that U.K. Prime Minister Theresa May has made clear is her top priority in the talks. Senior EU leaders have been adamant they will not discuss the future trading relationship with the U.K. until "sufficient progress" is made on the divorce terms for issues such as citizens' rights, border issues and a financial settlement.

But if Barnier was briefly willing to cast his gaze forward, it is unlikely that London will find much to celebrate in his crystal ball. The dapper Frenchman laid out a series of tough consequences for Britain and its businesses, making it clear that leaving the EU will hurt, and leaving the EU without a withdrawal agreement will hurt even more.

"I want to be very clear on this," he said. "'No deal' means a return to the status quo. In the case of Brexit, ‘no deal’ is a return to a distant past," including World Trade Organization rules that would impose "customs duties of almost 10 percent on vehicle imports, an average of 19 percent for alcoholic beverages, and an average of 12 percent on lamb and fish, for which the vast majority of British exports go to the EU."

Since the U.K.'s June election, which saw Prime Minister Theresa May lose her majority but cling on with support from Northern Ireland's Democratic Unionist Party, critics of the "no deal" scenario on the U.K. side have been more vocal, emboldened by what they see as the British public's rejection of a hard Brexit. In June, Chancellor Philip Hammond told the BBC: "No deal would be a very, very bad outcome for Britain."

May’s spokesperson said it was nothing new that some on the EU side had strong feelings about the decision the British people took last year, but London was focussed on getting the "best possible deal for our whole country and a deep and special partnership that is in the interests of the whole continent."

“We want a comprehensive free-trade agreement and a new customs agreement which allows for trade that is as frictionless as possible. Obviously we are just at the beginning of the negotiations, but I would say that the most frictionless possible trade between the U.K. and the EU is clearly in the interests of both sides.”

No cherry-picking

Barnier noted that the U.K. had set out a number of "red lines" for the future relationship, including no free movement of EU citizens; full autonomy over U.K. laws; the autonomy to conclude its own trade agreements; and ending jurisdiction of the European Court of Justice. He said those conditions meant leaving the EU's single market and customs union.

But at the same time, he said Brussels had laid out its own immutable positions: that free movement of people, goods and services were indivisible and a requirement of participating in the single market; that there could be no "sector by sector" cherry-picking, say for U.K. priority industries like automobiles or financial services; and that the EU must maintain regulatory sovereignty and "common standards."

"These three points were already made clear by the European Council," Barnier said, according to his prepared remarks. "But I am not sure whether they have been fully understood across the Channel."

He then sought to dispel any unrealistic expectations. "I have heard some people in the U.K. argue that one can leave the single market and keep all of its benefits — that is not possible," he said. "I have heard some people in the U.K. argue that one can leave the single market and build a customs union to achieve 'frictionless trade' — that is not possible."

Barnier added, "The decision to leave the EU has consequences. And we have to explain to citizens, businesses and civil society on both sides of the Channel what these consequences mean for them."

He also warned, "While leaving the customs union would in any case involve border formalities, ‘no deal’ would mean very cumbersome procedures and controls, without facilitation, which would be particularly damaging for companies that operate on a ‘just in time’ basis [such as perishable food producers]. For a manufacturer of sports equipment or industrial parts based in the U.K., whose products are at present shipped to the single market immediately, this would mean in practical terms, keeping their products in stock for three or four days instead of a few hours, renting warehouse space, an increase in transport costs, with a greater logistical risk."

"The best way of reaching an agreement is to create a climate of trust by tackling in the first instance three topics which we regard as priorities" — Michel Barnier

Barnier then effectively directed his remarks to London rather than his audience in Brussels, calling on the U.K. to swiftly settle the divorce issues so the talks could move ahead.

"The best way of reaching an agreement is to create a climate of trust by tackling in the first instance three topics which we regard as priorities and which are inseparable: citizens’ rights, the financial settlement and the border issue, especially the Irish border," he said.

Barnier continued: "I hope that rapid and sufficient progress will be made on these three topics together — and I mean on these three topics together — so that we can begin work as early as this autumn on the preparations for building a new partnership."

Annabelle Dickson contributed reporting from London. This article has been updated with a U.K. government response.