Theresa May’s Mansion House speech on Britain’s future relationship with EU was dismissed as vague and unworkable in Brussels, although the bloc’s chief negotiator welcomed the prime minister’s belated acceptance of new trade barriers for UK exporters to Europe after Brexit.

Michel Barnier said the UK’s clear confirmation that it will leave the single market and the customs union along with the prime minister’s “recognition of trade-offs” was at least a basis on which the EU could form its own position on the free trade deal to come.

May conceded in her speech that access to markets would be “less than it is now”.

On the substance of the prime minister’s proposals to reduce some barriers of trade and avoid a hard border on the island of Ireland, there was, however, deep scepticism among EU officials and diplomats who pointed to a speech by Barnier on Thursday in which he had already ruled out many of the prime minister’s suggestions.

Those proposals from May included a “comprehensive system of mutual recognition” under which goods need only be tested in the UK or the EU rather than in both territories to reduce costs and paperwork.

One senior diplomat echoed Barnier’s insistence that without EU law that can override national law, a common supervisory body and a common court, the suggestion “won’t work for us – or for that matter lead to frictionless trade”.

Aleš Chmelař, the Czech Republic’s state secretary for European affairs, said the UK’s demands remained unrealistic. “The UK says it does not want the obligations of Norway [to accept EU laws] and the [limited] rights of Canada. The impression in some member states currently is that the UK wants the rights of Norway and the obligations of Canada.”

EU diplomats involved in the shaping of the EU positions on Brexit were pleased by May’s suggestion that the UK would emulate Brussels laws in the future to maintain regulatory standards but queried whether the government could really commit to being a “rule taker”.

May speech fails to provide Brexit border solutions, says Irish PM Read more

However, it was the failure of the UK prime minister to offer any fresh thinking on avoiding a border between Northern Ireland and the Republic that received the harshest response.

May repeated a suggestion made last year that the government would mirror EU requirements on non-EU imports arriving in Britain and Northern Ireland that were destined for the rest of Europe, so there would be no need for a customs border between the UK and the union.

Alternatively, the prime minister said, there could be a technological solution that could make customs checks unnecessary on the island of Ireland and elsewhere.

One diplomat said the UK was simply not trusted enough for such arrangements to work. “The prime minister’s proposals are non starters,” he said. “Her customs idea is fantasy – we’d never do that. Mad cow disease originated on UK isles and [the country] is the biggest entry port for counterfeited goods.

“The hi-tech arrangement is sci-fi. No-one has seen it yet on Planet Earth. So back to square one on Ireland.”

May’s request for the UK to become “associate members” of EU agencies, such as the European Aviation Safety Agency, was additionally dismissed with the now familiar accusation that the UK was “cherry-picking” aspects of membership.

Guy Verhofstadt, the former prime minister of Belgium who is the European parliament’s Brexit coordinator, said: “Theresa May needed to move beyond vague aspirations, we can only hope that serious proposals have been put in the post.

“While I welcome the call for a deep and special partnership, this cannot be achieved by putting a few extra cherries on the Brexit cake.

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“Our relationship must be close and comprehensive, but this is only possible if the UK government understands that the EU is a rules-based organisation, as there is little appetite to renegotiate the rules of the single market to satisfy a compromise crafted to placate a divided Conservative party.”

Manfred Weber, leader of the largest party in the parliament, and a key ally of the German chancellor, Angela Merkel, said: “After what I have heard today I am even more concerned. I don’t see how we could reach an agreement on Brexit if the UK government continues to bury its head in the sand like this.”

EU officials did, however, welcome the warmer tone from the prime minister. “She played a straight bat and that doesn’t go down too badly on this side,” said one EU official.

The EU is due to publish guidelines on Tuesday on how it sees the future relationship with the UK, although they are expected to be short and uncompromising, repeating the red-lines that the prime minister is attempting to rub out.

A source said: “Whether the guidelines say it explicitly or implicitly, it will be like this in case the UK develops its position. We can then develop our thinking of what is possible.”