Nicola Sturgeon has declared it is hard to see how Boris Johnson's Brexit plans will 'fly', suggesting the proposal is only designed to fail.
Addressing the three-page letter sent today from the Prime Minister to European Commission president Jean-Claude Juncker, Ms Sturgeon was dismissive of hopes the proposals would yield a long-awaited breakthrough in negotiations.
Mr Johnson has urged the European Union to compromise to reach a Brexit deal as he set out his plan to scrap the Irish backstop.
READ MORE: Boris Johnson's Brexit plan revealed in full as Irish 'backstop' scrapped
In his letter, Mr Johnson said the backstop - the contingency plan agreed by the EU and Theresa May to prevent a hard border between Northern Ireland and Ireland - must be removed.
But the Scottish First Minister used Twitter to express her reservations over the proposals.
"Hard to see how the UK government Brexit ‘proposals’ fly," she tweeted.
"And hard to escape conclusion that they’re designed to fail.
"For Scotland, the fundamental point remains. These proposals would take us out of the EU, single market and customs union against our will. That’s unacceptable."
Brussels has also reacted coolly to Mr Johnson's blueprint to break the Brexit deadlock with a plan that would see Northern Ireland effectively remain tied to EU single market rules for goods, but leave the customs union.
Under his proposal, the arrangements would have to be approved by the currently suspended Northern Ireland Assembly, which would then vote every four years on whether to keep them.
In a letter to European Commission president Jean-Claude Juncker setting out his plan, Mr Johnson said he believed it represented a "reasonable compromise" to the issue of the Northern Ireland border.
READ MORE: Boris Johnson makes fish gags about Nicola Sturgeon in Tory conference speech
However, after speaking to the Prime Minister by telephone, Mr Juncker said while there had been some "positive advances" there remained "problematic points", particularly over the "governance of the backstop", which needed to be resolved.
In a statement, the commission said Mr Juncker welcomed the proposals for "full regulatory alignment" between Northern Ireland and the EU.
"However, the president also noted that there are still some problematic points that will need further work in the coming days, notably with regards to the governance of the backstop," the statement said.
"The delicate balance struck by the Good Friday agreement must be preserved."
The statement also expressed concern about the proposed customs rules, saying they still needed a "legally operational solution" to ensure there was no return of a hard border on the island of Ireland after Brexit.
In his letter, Mr Johnson said failure to reach an agreement ahead of Britain's scheduled withdrawal from the EU on 31 October would represent "a failure of statecraft for which we would all be responsible".
"Both sides now need to consider whether there is sufficient willingness to compromise and move beyond existing positions to get us to an agreement in time," he said.
"We are ready to do that, and this letter sets out what I regard as a reasonable compromise: the broad landing zone in which I believe a deal can begin to take shape."