Speaking on France Inter radio, Barnier said a deal to keep goods flowing smoothly between EU member Ireland and Northern Ireland in the U.K. after Brexit is a "prerequisite" for a successful outcome to the divorce talks.
Barnier said avoiding such a "hard border" is a "condition for peace and stability for this island following many tragedies" and that the issue has the potential for the talks to end in failure. The Irish border will be U.K's only land frontier with the EU after Brexit. Both sides agree there must be no hard border that could disrupt businesses and residents on both sides and undermine Northern Ireland's hard-won peace process. But each has rejected the other side's solution.
The EU has suggested that a back-stop solution is to keep Northern Ireland inside a customs union with the bloc, but Britain rejects that because it would mean customs checks between Northern Ireland and the rest of the U.K. Britain has proposed instead that all of the U.K. could stay in a time-limited customs union. The EU insists there can be no time limit.
"I don't have an intimate conviction on this matter because the political situation is extremely complex in the United Kingdom," Barnier said. "And I do not know what decision (Prime Minister) Theresa May will take. I want an agreement; I'm working on it because it's the common interest."
Britain is set to leave the bloc on Mach 29, 2019. If there's a deal, then a transition period to smooth the Brexit process will see Britain staying in the European single market and customs union even though it's outside the EU's decision-making institutions. May has suggested that any transition could be extended, albeit for only a few months beyond the end of 2020 as is currently proposed.
In Brussels, Irish Prime Minister Leo Varadkar said that concerns about a possible return of past tensions — about 3,700 people died during 30 years of "troubles" — cannot be ignored. "Certainly it's something that's in the back of my mind as a potential consequence if we get this wrong," he told reporters.
Varadkar also it's not just a question of satisfying May's allies and the U.K. parliament. "This isn't just about getting a deal that Westminster can accept. It also has to be accepted by the European Parliament as well, and the European Parliament has been very strong in supporting Ireland and our need to have a backstop to avoid a hard border."
After leading Brexit-backers in the U.K. accused the EU of "bullying" this week, Barnier insisted his negotiating team have not sought out to punish Britain. "I have a great admiration for this country, we have a debt towards the United Kingdom for the solidarity they have always shown, even during the most tragic times of the 20th century," Barnier said.
Lorne Cook in Brussels contributed to this report.