EUROPEAN UNION chief Brexit negotiator Michel Barnier issued new threats of a “hard border” with Ireland today if Britain leaves the single market.

The former EU commissioner also threatened to derail talks on a two-year “transition” period after Britain’s withdrawal from the bloc becomes final in May next year.

Following the latest round of talks with British Brexit Secretary David Davis, Mr Barnier said three “substantial” disagreements remained, warning: “if these disagreements persist, the transition is not a given.”

Claiming he was “surprised” by Downing Street’s position on the customs union Mr Barnier said Britain had to face the “consequences” of reasserting its sovereignty.

“A UK decision to leave the single market and to leave the customs union would make border checks unavoidable,” he said.

Almost a million Irish citizens live and work freely in Britain, while there are almost 300,000 Britons in the republic.

The EU had insisted that Northern Ireland must remain within the bloc’s “regulatory framework,” with a “hard border” down the Irish Sea — a demand that, if granted, seems likely to destroy the Tories’ alliance with the Democratic Unionist Party.

Communist Party of Ireland general secretary Eugene McCartan said: “The Irish border is a useful foil in the sham fight that is taking place at this time,” pointing out the “dominant elements of the British ruling class” and state want to stay in the EU.

“The Tories can't afford to upset the DUP at the moment thereby provoking a general election in Britain,” he said.