BRUSSELS could reject Theresa May's plan to safeguard the continuation of a soft border between Northern Ireland and the Republic, Ireland's foreign minister has warned.

The Prime Minister has committed to leaving the EU customs union which guarantees tariff-free trade but insists a hard border can be avoided through technological solutions and placing no new restrictions on the 80 per cent of cross-frontier trade carried out by smaller businesses.

But Simon Coveney, the Irish Tanaiste, said he was "not sure that the European Union will be able to support" the plan as it would be worried about protecting the integrity of the single market.

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He told BBC One’s Andrew Marr Show: "While, of course, we will explore and look at all of the proposed British solutions, they are essentially a starting point in negotiations as opposed to an end point."

Mr Coveney said if agreement could not be reached during planned tri-partite talks between the UK, Ireland and the European Commission, the “backstop” plan of full British alignment with customs union and single market rules that Mrs May "committed clearly" to in December would have to be put in place.

In a major speech on Friday, Mrs May rejected "unacceptable" EU proposals to retain customs union arrangements in Northern Ireland but accepted the UK's "responsibility" to help maintain a soft border with the Republic, spelling out in detail how she believed this could be achieved by technological means or through a broader trade agreement.

However, Mr Coveney said: "This isn't a question of either side wanting to put up borders but if you have to protect a functioning single market, just the same way Britain wants to protect its own single market, well then you have to understand that if goods move from one customs union to another, then there needs to be some checks unless there is some mechanism that is negotiated and put in place that prevents that."

Mrs May said she was pleased that Irish PM Leo Varadkar had agreed to form the three-way talks to look at her proposals.

When asked, she declined to defend Foreign Secretary Boris Johnson's comparison of the border to crossing between London congestion zones in Camden and Islington but insisted both of them were "absolutely clear" that there would not be a hard border.

"We've got proposals as to how we're going to achieve that, now we're going to be able to sit down and talk with others about how we're going to do that," the PM told Mr Marr.

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It comes after Downing Street and Mr Johnson denied reports that Mrs May's chief of staff Gavin Barwell was behind a leak of a memo from the Foreign Secretary in which he said the Government should focus on stopping the Irish border becoming "significantly" harder, reigniting a row over the issue.

Meanwhile, Lord Mandelson, the former Labour Business Secretary, dismissed Mrs May's Brexit plan.

"What Theresa May is doing is trying to dance on the head of a pin that simply doesn't exist. It will be painful for the country as a result," he said.