Michel Barnier, the EU's chief Brexit negotiator | Paul Faith/AFP via Getty Images Barnier: Brexit checks between Britain and Northern Ireland ‘indispensable’ ‘Brexit unfortunately has consequences that we must manage,’ says EU negotiator.

Customs checks between Great Britain and Northern Ireland will be “indispensable” after the Brexit transition period ends in December, Michel Barnier said today.

Speaking at Queen's University Belfast, the EU’s chief Brexit negotiator pushed back against Boris Johnson’s repeated claims there will be no checks on goods crossing the Irish Sea after Brexit.

Barnier insisted the Northern Ireland protocol Johnson signed up to in October “makes frictionless trade impossible. It makes checks indispensable.”

He said: "I understand the fears of negative economic fallout expressed by some about these checks. But Brexit unfortunately has consequences that we must manage."

Barnier argued that the EU will never compromise on the security of the single market, and said that as a result “there will be no possibility for frictionless trade” between the EU and the U.K. after Brexit.

Stating that the Withdrawal Agreement was an international treaty which had been ratified by the House of Commons and was about to be ratified by the European Parliament, he said: “We have to respect this text in all its dimensions.”

The agreement effectively keeps Northern Ireland in the customs union while allowing it to benefit from new trade deals signed by the EU. It means goods that go to Northern Ireland and could cross the border in the Republic of Ireland must be checked to ensure they comply with EU rules.

Meanwhile, Barnier also warned that if the U.K. refuses to play ball on competition rules it could lose EU market access. “Aligning with our standards may have a price. But the return is access to our Single Market,” he said.