THERESA May has suffered a further setback in her bid to sell her post-Brexit customs plan to the EU27 after another senior European politician suggested it was unworkable.

On Thursday, Michel Barnier, the EU’s chief negotiator, was said to have “blown up” the centrepiece of the Prime Minister’s Chequers Plan - using the facilitated customs arrangement whereby the UK and EU collected each other’s tariffs at the border – when he made clear the bloc would not allow a non-member to handle its tariffs.

Today as Mrs May flew to Salzburg for talks with Sebastian Kurz, the Austrian Chancellor, as well as Andrej Babis, her counterpart from the Czech Republic, his colleague, Ales Chmelar, the Czech State Secretary for European affairs, also made clear the PM’s plan was unacceptable.

Backing Mr Barnier's position, Mr Chmelar told BBC Radio 4's Today programme: "There is a clear problem with the fact that the EU will not have a mechanism to control its borders and it would be delegated - without any EU control - to a third country, which would be Britain after March.

"So, that is the key principle, but also there is a larger general principle... it is almost physically impossible to have, at the same time, full regulatory autonomy on one side and full market access.

"There needs to be a certain balance between the rights and obligations in those terms."

The European Commission has also underscored the EU27’s objection to the Chequers Plan for future customs arrangements, tweeting: “The UK wants to take back control of its money, law, and borders. We will respect that. But the EU also wants to keep control of its money, law, and borders. The UK should respect that.”

Mr Chmelar made clear any hope Mrs May might have of finding support from her counterparts in Salzburg today could be misplaced.

While admitting the Czechs had some concerns over the way the EU worked, he insisted the bloc remained united in its approach to Brexit talks and there would be no “loopholes” to exploit.

"The fact that we are maybe critical of some aspects of EU policies, be it in migration or be it in other areas, does not meant that we wouldn't stand behind a very strong position on the integrity of the single market.

In a summer charm offensive when various ministers will visit their counterparts on the continent to sell the Chequers Plan, the UK Government will seek to highlight the potential impact of the failure to reach a Brexit deal on individual economies across the bloc, hoping that domestic political interests might result in pressure from EU national leaders on the Brussels bureaucracy to soften its stance.

However, Mr Chmelar said the EU's leaders had been fully involved in approving the mandate given to Mr Barnier and stressed that, while the UK was an important trading partner, it was not as vital as the Czech Republic's neighbouring countries.

Yet he acknowledged that "every member state of the EU is, to a certain extent, worried" about the prospect of a no-deal.​