Theresa May’s chances of securing a deep free-trade deal with the EU were dealt a blow when Stefaan de Rynck, the main adviser to the EU’s chief Brexit negotiator, Michel Barnier, stressed that the rules of the single market required far more than her chief proposal – a mutual recognition of standards.

May claimed in her speech last Friday that the UK could negotiate a future trade relationship based on mutual recognition of standards overseen by a third party court, made up of EU and UK nominees.

But De Rynck said: “The EU has moved away in the wake of the financial crisis from mutual recognition of national standards to a centralised approach with a single EU rule book and common enforcement structures and single supervisory structures.”

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He added that EU rules were clear that the European court of justice could intervene at any point to declare that mutual recognition of standards was undermining the single market’s integrity.

Such principles were neither empty or legalistic dogma, he said, but fundamental to the integrity of the single market and autonomy of the EU.

His remarks, at a special LSE lecture in London on Monday night, are probably the fullest from the European commission since May made her speech and suggest her proposals for a future trading relationship will be dead on arrival, unless EU member states sympathetic to UK interests decide the commission’s interpretation of the single market is too rigid.

De Rynck insisted there was no sign of division among the EU 27 member states or European institutions, warning the UK that “it would be very unwise to break down that unity”.

He also claimed EU businesses, faced by a choice, “are more concerned with maintaining the integrity of the EU single market than any loss of access to British markets”, implying EU business is in no mood to press the commission to compromise in the talks.

He also warned the UK that there was no appetite to extend the talks on UK’s exit beyond the current timetable of March 2019, and if the UK was to seek to rescind its proposals to withdraw from the EU set out out in its article 50 letter, this would not be just be a matter for a UK unilateral move, but require collective response by EU member states.

De Rynck said there was little precedent for third parties, including the UK after Brexit, to secure participation in key EU agencies, such as the European Medical Agency, saying the only three exceptions were countries such as Norway that were members of the European Economic Area Agreement (EEA). The UK has ruled out being an EEA member since it would require the UK to accept the free movement of people.

He also defended the commission from criticism that its proposals, unveiled last week setting out a fall-back position on the future of the north-south Irish border if no other agreement could be reached, represented an over-reach by the EU or an infringement of UK sovereignty.

He said the full legal text prepared by the commission on the fall-back option was necessary since it would have been “complacent for the EU to leave such a sensitive issue as a backstop solution to the very last moment of the negotiations”.

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He pointed out that there had been a joint EU-UK commitment last December to defend the Good Friday agreement, adding the EU 27 was heavily invested in the Irish peace process.

He added it was a fantasy to suggest that it was the EU that was seeking to impose a north-south border, saying such a border was a legal necessity for the UK, and was a product of the UK decision to leave. He said “Who is asking for a border? Brexit is asking for a border”.

He refused to speculate on how long the transition might last, or if it would extend beyond December 2020, saying the transition could only be agreed once the withdrawal agreement was settled.