THERESA May’s Chequers Plan has been declared “dead” by pro-EU campaigners after Michel Barnier blew a big hole in its centrepiece customs proposals.

Key to the Prime Minister’s policy is the "facilitated customs arrangement," whereby tariffs charged at the border would be passed on to either the British or EU authorities depending on the destination of imported goods.

But the EU’s chief negotiator, standing alongside Dominic Raab, the Brexit Secretary, left no doubt that the UK Government’s customs blueprint was totally unacceptable to the EU27.

Speaking to a press conference following their second round of talks in Brussels, Mr Barnier declared: "The EU cannot and the EU will not delegate the application of its customs policy and and rules and VAT and excises duty collection to a non-member, who would not be subject to the EU's governance structures."

However, he stressed how the EU was “open to a customs union” with the UK after Brexit; something that is anathema to Mrs May and her colleagues as it would prevent Britain from securing future trade deals with other countries.

The European Commission underscored Mr Barnier’s point, 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.”

The Brussels broadside came as Mrs May prepares for a trip today to the Salzburg music festival, where she will have talks on her Brexit plan with Austrian Chancellor Sebastian Kurz and other EU leaders. It is likely to be her last bilateral before heading off for a summer walking holiday in Italy and Switzerland with husband Philip.

Pro-EU campaigners insisted the European Commission had “in one fell swoop…blown up” the PM's post-Brexit customs plan.

Labour’s Virendra Sharma for Best for Britain said: “Barnier has savagely slapped down the Government's already broken Brexit plan…It's incredibly embarrassing having to be told, once again, that the key element of your proposal is unworkable…It's dead."

His Labour colleague, Ben Bradshaw, the former Cabinet minister, also claimed Brussels had “killed the White Paper”.

He said: “Mrs May’s scheme could not command a majority in the Commons two weeks ago when she allowed the Brextremists to mutilate it. Now we know that hardly mattered because it was not acceptable to the EU in any case.

“There is only one way out of this mess and that is to have a People’s Vote on the final outcome of the Brexit negotiations.”

Christine Jardine for the Liberal Democrats noted: “The EU has made it explicitly clear that it will not delegate responsibility for its customs policy, VAT and tariffs to a non-member. This leaves the Prime Minister’s deal struggling to survive.”

The Edinburgh MP added the political momentum was now moving towards another referendum and giving voters the opportunity to “exit from Brexit”.

In another slapdown to the UK Government, Mr Barnier also made clear that, irrespective of whether there was a deal, Britain should pay up the agreed £39 billion divorce settlement. Earlier this week, Mr Raab made clear: no deal, no money.

But the EU’s chief negotiator said that, while the commitment to a financial settlement made by Mrs May in December was not yet in its final legal form, the EU27 and the European Parliament regarded it as "agreed for good".

Mr Barnier also made clear that Brussels still had reservations about the UK Government’s "backstop" proposal for the Irish border, which would see the whole UK matching EU trade tariffs for a period if a trade deal were not reached by 2021.

Despite the Brussels knock-backs, Mr Raab appeared upbeat, signalling that he was looking for compromise from Brussels in response to Mrs May's White Paper, noting how the EU was able to take an "innovative" approach "when the political will has been there".

He added: "With ambition and pragmatism and energy on all sides, we can get there in October."

Earlier, Boris Johnson aimed another barb at the PM’s Brexit plans, branding them unworkable and calling on fellow Tory MPs to "chuck Chequers".

The former Foreign Secretary - who walked out of the Cabinet days after signing up to the package at the PM's country residence – claimed Mrs May's blueprint would leave the UK in "vassalage, satrapy, colony status" to the EU.