THERESA May’s proposal for a Brexit deal with the European Union has been blasted by trade experts as “fanciful” and based on flawed analysis.

The Prime Minister’s deal, known as the Chequers plan, made a pledge that the vast majority of businesses would pay the right or no tariff at the border but this has now been thrown into doubt.

Experts have also questioned another key part that businesses would be able to reliably track goods to their final destination.

This aspect is particularly important to the EU which has expressed concerns about smuggling in the continent.

Analysis of the proposals has been carried out by Alan Winters, professor of economics and director of the UK Trade Policy Observatory at Sussex University.

He said: “The whole thing when you analyse it is pretty fanciful.”

Should her proposal collapse it would increase the chances of having to choose between remaining within the customs union but limiting free trade deals or completely pulling out.

Number 10 said the Chequers proposal corrected flaws in the original idea, which was blasted for being costly for business to implement and undermining the principles for free trade.

Professor Winters told The Times: “It is weird that they are using the whole of trade for the basis of their calculation when it is clear that it is only imports that will be affected and it has nothing at all to do with exports.

“The idea that you would know where all finished goods being imported were headed also doesn’t make sense.

“This is clearly reasonable for some goods like meat that are heavily regulated, but we cannot see it applying automatically to all goods.”

Anna Jerzewska, an expert in trade agreements, said the government’s assumptions appeared to have been “exaggerated at best”.

“I don’t know any business that is routinely able to determine where all the goods they import end up and in which market. You simply don’t have that visibility,” she said.