Sir Ivan Rogers, who resigned over claims ministers ignored his advice on Brexit, used the example of Scottish whisky to argue that exports to Europe would face harsher trade terms and more competition, regardless of any deal.
SNP MP Joanna Cherry said the comments were a “devastating critique” of Scottish Tory leader Ruth Davidson’s for the UK to seek the “greatest possible access” to the EU market.
Highlighting Ms Davidson’s previous support for full single market membership, Ms Cherry said: “The Scottish Tories have been all over the place on access to the single market.”
She added: “Using the example of Scottish Whisky, we were told that access to the single market means Scotch can be sold; while membership means all tax and duties on comparable products to Scotch must be the same throughout the market.
“Using the example of aviation he said that with access to the single market planes can land, and return membership means the planes get a slot, gate and lounge allocation on the same terms as local airlines and they can fly within EU and not just to and from.
“As Sir Ivan’s evidence has highlighted today, the day after an exit from the EU would be radically different in the absence of any agreement - or as he put it - exiting the EU without membership of single market, and without any deal, would mean stepping into a legal void In areas which would do the UK massive damage.”
A Conservative spokesman responded: “If Joanna Cherry is so concerned about maintaining market access, she must explain why she wants to separate from the rest of the United Kingdom – a market worth four times as much to us as the rest of the EU.”
Sir Ivan also said the Scottish Government’s plan to stay in the single market after Brexit was “legally creative” and would be opposed by EU members.
"I would think that the usual suspects in some of the usual member states would be extremely worried about the precedent - so you start then with Spain, but you probably have Belgium and Italy as well, who will be worried about the implications for their jurisdictions and the unity of their jurisdictions if you ever get into differentiation," he said.
And Sir Ivan warned warned the EU is unlikely to agree specific deals for industries like car manufacturing which are highly dependent on Europe-wide supply chains, despite pressure from the likes of Germany's automotive industry.
He pointed out that German Chancellor Angela Merkel has repeatedly said "no sectoral interest will be put above the general interest".
"Well, with Merkel - if she's still there - the unity, the unity of the 27, will win out and I think she and others will agree that there will be no sectoral deals in either the single market or the customs union, and I expect that to appear in either the guidelines or the negotiating mandate," he told MPs.
Liberal Democrat former cabinet minister Alistair Carmichael urged Mrs May to admit the best option for the UK is to remain in the single market.
"Theresa May and her Brexiteers are living in La La Land," he said. "They are promising special deals to key sectors that are unlikely to ever be delivered."