In a move that casts doubt on the UK's ability to leave the customs union post-Brexit, senior EU sources briefed the Telegraph that Theresa May's border plan was this week subject to a "systematic and forensic annihilation" at a meeting in Brussells this week.
The "Irish question" of how Britain can balance an open Irish border while pulling Northern Ireland from the customs union and single market increasingly appears to be the one issue which could bring the Brexit process screeching to a halt.
"There's no cherry picking in the single market". Pressure to this effect is mounting in London, as Theresa May was forced to deal with a House of Lords demand of the United Kingdom government exploring "a Custom's Union" deal during Brexit negotiations, in line with Labour's position.
"And, after a stinging defeat in the Lords earlier this week and every sign that the Commons will next week also back continued Customs Union membership, it is also clear that it is not just the European Commission who have no faith in her attempts at sleight of hand".
Labour peer Andrew Adonis added: "The Government is slowly waking up to the fact that the Brexit they have promised is not a Brexit that is possible".
Mrs May's plans did not stand up to a "systematic and forensic annihilation" by European Union mandarins during the fifth round of technical negotiations this month, according to the Daily Telegraph.
Mrs May will reportedly gather her "war cabinet" for weekly meetings as she tries to figure out a new solution to the border and customs union issues.More news: Procter & Gamble buying Merck's health brands for $4.2B
A spokesman for the Prime Minister said "we don't recognise these reports", but stopped short of saying they were untrue.
Three options have been proposed by British officials to avoid having a hard border: a comprehensive free trade deal and customs agreement that would make customs checks unnecessary, technology such as number-plate recognition cameras, and, most controversially, regulatory alignment with the Irish republic as a "backstop".
The second, a customs arrangement, envisages technological and administrative measures ensuring that trade with the European Union remains frictionless, with additional measures for Northern Ireland such as allowing small traders to operate across the Border unchecked.
"We have agreed that the areas covered in the draft must reflect those that meet our shared commitments".
Responding to the Telegraph report, former Treasury permanent secretary Lord Macpherson tweeted: "EU's position on Irish border so predictable".