Theresa May has lashed out at internal Conservative critics of her plans for the Irish border, accusing them of being ready to “betray” the people of Northern Ireland and the Republic.

And she sent a message to Brussels that the EU must change its negotiating position in response to the plans for a post-Brexit relationship which she drew up at Chequers.

The Prime Minister was speaking in Belfast as ministers from the remaining 27 EU states met in Brussels for a briefing from chief Brexit negotiator Michel Barnier on the plan set out in Mrs May’s White Paper last week.

In a sign of growing concern in Dublin about the prospect of a hard Brexit, Irish Taioseach Leo Varadkar suggested Ireland could close its airspace to UK planes if Britain seeks to ban EU ships from fishing in its waters.

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Prime Minister Theresa May urged the EU to ‘evolve’ its position on Brexit and not fall back on ‘unworkable’ ideas (Charles McQuillan/PA)

In her first major Brexit speech since the wave of ministerial resignations which followed her Chequers deal, the Prime Minister described the White Paper proposals as “a significant development of our position … a coherent package”.

And she said: “It is now for the EU to respond – not simply to fall back on to previous positions which have already been proven unworkable, but to evolve their position in kind.

“And, on that basis, I look forward to resuming constructive discussions.”

She sent a blunt message to supporters of a hard Brexit, like Jacob Rees-Mogg, who have argued that the UK should simply declare it will impose no checks at the Irish border after EU withdrawal and leave it to Brussels to decide whether to require the Republic to erect barriers.

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Prime Minister Theresa May met members of the Belfast Youth Forum before her speech (Paul Faith/PA)

“This issue arises because of a decision we have taken,” she said. “We can’t solve it on our own, but nor can we wash our hands of any responsibility for it, so we must work together to solve it.”

The UK has a “duty” to ensure that its borders with neighbouring countries function smoothly, she said, adding that this was “a particular challenge” in Northern Ireland.

“The protection of the peace process and upholding our binding commitments in the Belfast Agreement are grave responsibilities,” she said.

“Not to seek a solution would be to resume our career as an independent sovereign trading nation by betraying commitments to a part of our nation and to our nearest neighbour.”

And she took a swipe at former foreign secretary Boris Johnson’s claim – repeated in his resignation speech to the Commons on Wednesday – that technological solutions could be used to avoid the need for infrastructure at the border.

“No technology solution to address these issues has been designed yet or implemented anywhere in the world, let alone in such a unique and highly sensitive context as the Northern Ireland border,” she said.

Mrs May restated her implacable opposition to the European Commission’s proposed “backstop” arrangement which would see Northern Ireland remain within the EU customs union.

This would involve the creation of a customs border within the UK, which was “something I will never accept and I believe no British Prime Minister could ever accept”, she said.

Equally, she said that a hard border between Northern Ireland and the Republic was “almost inconceivable”. Twenty years after the Good Friday Agreement, the return of any form of physical checkpoints or other infrastructure would be “an alien concept”.

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(PA Graphics)

Mrs May said that her White Paper proposals, which would see the UK remain within the single market for goods and adopt a “common rulebook” of regulations with the EU, represented a “credible third option” that would “honour the Belfast Agreement, deliver on the referendum result and be good for our economy”.

As EU27 ministers gathered in Brussels, there were signs of concern that the recent turmoil at Westminster might make it more difficult to achieve an orderly withdrawal.

German Europe minister Michael Roth said it was “hard to be an optimist” with the political difficulties in the UK, adding: “Time is running out, the clock is ticking and that’s why I am a little bit nervous.”

French European affairs minister Nathalie Loiseau said: “We will work on the basis of our principles and see to what extent our British partner fully gets it, including the British Parliament.

“We know that there have been amendments to different provisions in Britain which makes it even harder for us to discuss with our British partner.”

Gernot Blumel, the Europe minister of Austria, which currently holds the presidency of the European Council, told reporters in Brussels: “A ‘hard Brexit’ is not off the table, but we are naturally trying to do everything we can to arrive at a solution and a course of action.”