Says Jean-Claude Piris, former head of the EU Council's legal service, it will be "totally impossible" for Britain to wrap up a trade pact with the EU within two years. Such a deal would comprise "thousands of pages and hundreds of articles" and there was no chance of it being ready before a scheduled Brexit in 2019.
Mr Piris says that a UK-EU trade deal "could take up to 10 years" because of the complexity of the task, but added that he hoped it could be concluded more quickly. "We could do it in maybe five years, I don't know".
"The important thing", he adds, "is that clearly there is a gap of a few years between the date where the UK is withdrawing from the EU and the date where this agreement comes into effect, perhaps five or six years".
Piris calls this the "WTO gap", when we would have to fall back on WTO rules as the basis for our trading arrangements. And that, in his view, would be "a catastrophe for the EU" but mainly for Britain because it did nearly half of its trade with Europe. To avert that catastrophe, he says, the UK will need a transition deal.
However, when it comes to working out the detail, Piris is less than clear. He suggests that there are [only] two kinds of transitional deal that might be used to bridge the gap, the first of which would see Britain staying in the single market for a few years.
This, rather unfortunately, would require Mrs May to sign up for a limited period to existing obligations including the jurisdiction of the European Court of Justice and free movement of people - both of which she has said are red lines in her negotiation.
The second option Piris has on offer is "much more modest" (his own definition) where Britain negotiated to stay in the customs union, similar to the deal enjoyed by Turkey.
"That would mean the territory of the UK would be in the customs union and you benefit from the whole of the EU's trade policy including all the agreements we have done in the past and the ones we are going to conclude soon with Japan and so on", he says.
Piris is slated as "one of the EU's most eminent lawyers", which goes to show that even prestige cannot prevent people getting it wrong. He of all people should know that Turkey is not in the EU's customs union, and neither would staying in it (if we could), give us access to the EU's external trade deals.
Whatever Piris might think, there is no scenario where negotiating a customs union with the European Union could serve adequately as a transitional arrangement. Only some means of continuing in the Single Market would suffice, which is best achieved by the Efta/EEA option – something Piris does not mention.
Nevertheless, the man is not alone in his predictions of "catastrophe" for the WTO option. He is effectively supported by Matthias Wissmann, head of the German automobile industry association.
Writing of the forthcoming negotiations, he argues that the core question remains unresolved: how does Britain manage to end EU membership on the one hand, and on the other hand to continue to benefit from the country's previous EU membership?
Is it realistic, he asks, to choose only three from the four freedoms of the EU - free movement of persons, goods, services and capital, while limiting free movement of persons? Brussels, he says, has already made it clear that the "four fundamental freedoms" cannot be put at risk.
Yet, he says, there is a deep reciprocal dependency between Great Britain and the EU as well as Germany. And from a German perspective, the British are also strategically important: they are the voice of market economy, competition, as opposed to representatives of a "transfer union". Without London it would be even more difficult for Berlin in Brussels to stand up against the other EU countries.
Wissmann thus declares that a hard Brexit would be a "mission impossible". The EU and the UK would face "massive negative effects" if Britain left the Single Market.
This is more or less a repetition of his warning in October, when he was suggesting that car makers were likely to move production to low-cost EU countries in eastern Europe if the UK was no longer part of the Single Market.
Currently, though, Wissmann thinks that the Brexit talks must centre on the UK remaining in the internal market and in the customs union. It should accept the basic freedoms and make a financial contribution to the EU budget in return for unimpeded access to the internal market.
What thus we seem to be seeing is Continental figures playing catch-up – still years behind and not yet focused on the issues, telling us things that should not have to be said.
But, for all the determination of the chatterati to ignore the Efta-EEA option – or to damn it with faint praise – it isn't going away any time soon. In a welcome intervention, George Peretz argues that it is now up to our democratically elected politicians to decide whether the EEA is the right option.
Of course, the government should be mindful of whether that delivers what the electorate, or at least the bulk of it, wants (or is prepared to live with as part of a compromise).
But, he says, that electorate includes not only the unknowable number of "leave" voters for whom the EEA delivers what they wanted, but also the over 48 percent who voted to stay in the EU, and who can reasonably be supposed to prefer that option to options that seek to sever the UK further from the rest of the continent.
Politicians may – properly – decide that, on its merits, the EEA is not the right way forward. But that argument on the merits cannot properly be shut down by false claims that that option is foreclosed by the 23 June vote.
As a matter of basic constitutional principle, Peretz concludes, it is the job of elected politicians to represent, and take into account the views of, all their voters - not just the subset who happened to agree with Vote Leave's views on the single market – whatever they were.
Gradually, I think we are seeing a merging of ideas, which may eventually get us there, freezing out the zombies who would have us crash the economy just to sate their stupidity. The "transitional deal" is definitely on the agenda. All we need now is a consensus that Efta-EEA satisfies this need.