Barnier said Brexit would trigger the need for customs, Value-Added Tax and compliance checks with EU standards between Ireland and Northern Ireland in the event that a planned "backstop" were triggered because a future EU-UK trade deal was not sufficient in itself to ensure the land frontier was not a "hard border".
"The prime minister is a unionist".
He also stressed the EU's insistence that Britain must accept possible checks on goods moving between its mainland and its province of Northern Ireland, saying Brexit will trigger the need for customs, value-added tax and compliance checks with European Union standards.
The news comes after the Democratic Unionist Party threatened to vote against the Budget, and potentially bring down the Government, in opposition to a backstop plan that would treat Northern Ireland differently to the rest of Great Britain. Many of her cabinet colleagues have assured me of their Unionism.
"Therefore, they could not in good conscience recommend a deal which places a trade barrier on United Kingdom businesses moving goods from one part of the Kingdom to another".More news: Will approach both friendly countries, International Monetary Fund to overcome economic crisis: Imran Khan
The EU suggests Northern Ireland stay aligned with its customs union and single market during that period, and there are signs the British government could accept elements of this plan.
EU leaders are due to meet for dinner in Brussels next Wednesday and hope to agree a withdrawal treaty with Britain that Barnier said was 80-85 per cent ready but on which obstacles remain, notably on how to keep the new EU-UK land border with Northern Ireland from reviving conflict in the British province.
On Wednesday evening, DUP MPs abstained in a minor agricultural vote in Westminster to show they are not afraid to strong-arm the Government over proposed plans for the Irish backstop.
May's Conservative party has relied on the DUP's 10 MPs to pass legislation since losing its majority in the House of Commons in the June 2017 election.
The party's Brexit spokesman Sammy Wilson later explicitly warned the Government against doing a deal that kept Northern Ireland in the Single Market, writing in the Telegraph the party could not support "any deal which includes such economically and constitutionally damaging arrangements".
"Without an end date, we could be in the customs union forever".