MICHEL Barnier has dismissed Theresa May’s proposals for future customs arrangements casting doubt over whether they would be workable, in his first public response to last week’s White Paper.

Speaking after a three-hour briefing with foreign ministers from the 27 remaining European Union states in Brussels, the EU chief negotiator said there were elements of the plan agreed by the Cabinet which the European Commission did not understand.

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He said May’s plans for a “facilitated customs arrangement” opened up the risk of major fraud, additional bureaucracy and damage to EU businesses and that further discussions would be needed to establish how much “common ground” exists between London and Brussels.

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Barnier was speaking shortly after May issued a challenge to Brussels to “evolve” its negotiating position in response to the publication of her Brexit blueprint.

Responding to her comments, made in Belfast, Barnier said the EU had always been “creative and flexible” in its approach to negotiations, but insisted future talks would be based on the guidelines issued by the European Council in March. These included the proposal to keep Northern Ireland in the EU customs union and single market – and not on May’s document.

“There will be a deal if there is an agreement on the backstop,” said Barnier. “It’s not necessarily our backstop. We can work on this, amend it, improve our backstop – the one that the Commission proposed on behalf of the Union. Technically we feel that it’s workable, we can improve it further, we can work on it. We are doing that work. But we need an operational backstop now, in the Withdrawal Agreement, and not later.”

A senior EU diplomat said member states had been surprised and disappointed by the aggressive tone of May’s speech, in which she accused the EU of seeking to perform an “economic and constitutional dislocation” of the UK.

And Barnier indicated he believed the UK’s position appeared far from settled.

“This White Paper is the result of a very intense debate in the United Kingdom that was necessary. Everyone can see that this debate is not over,” he said.

Setting out the questions he had posed to the new Brexit secretary, Dominic Raab, during their first meeting on Thursday, he questioned whether a non-EU country could collect EU customs without being subject to EU oversight. For this reason alone, EU diplomats say privately that the British plan could never be accepted.

The EU is deeply concerned the customs plan would give outsider firms a competitive edge over European rivals if the British and other countries use the UK as a route to avoid higher EU tariffs. “Are the British proposals in the interests of the EU?” Barnier asked.

With reference to a EU position paper, which ruled out so-called “cherry picking” of elements of bloc’s membership, Barnier said he believed the UK had breached the EU’s red lines.

“There are some elements [of the White Paper] which do seem to contradict the guidelines of the EU council... namely the indivisibility of the four freedoms and the integrity of the single market,” he said.

On Monday, May accepted four amendments put forward by Jacob Rees-Mogg, the chair of the European Research Group of MPs, including one making a border in the Irish Sea illegal. Arriving in Brussels France’s European affairs minister, Nathalie Loiseau , said the acceptance by May of those amendments had further complicated the talks. She said: “We know there have been amendments to different provisions in Britain, which makes it even harder for us to discuss with our British partners.”