Raoul Ruparel, who served as Mrs May’s special adviser on Europe from 2018 to 2019, suggested Brexit Britain had missed out on a surge in demand from the United States. Such demand has also assisted several other exports.
US demand for imports surged last year after US President Joe Biden continued to bring in high levels of fiscal stimulus.
American importers are said to have turned to electronics and metals.
However, Mr Ruparel claimed the UK is a “very minor” exporter in such sectors.
Mr Ruparel, who now works at Deloitte, also said the shortfall is “not entirely down to Brexit and likely not even mainly down to Brexit”.
He added: “By far the larger factor is the poor performance of UK exports to non-EU countries, especially the US.
“This is partly because the UK found itself poorly placed to take advantage of the increased demand since it seemed to largely fall in areas where the UK is not a major exporter.”
British exports are around 12 percent lower than pre-pandemic levels, according to the Telegraph.
In comparison, global goods exports are about 10 percent higher and 5.5 percent higher in advanced economies.
However, Britain did witness an increase in demand in the chemicals sector, according to Mr Ruparel.
But exports in the chemical sector could only return to their 2016 levels.
Ruparel said: “It is possible this is partly due to additional Brexit impacts with the sector as a whole becoming less competitive due to additional costs and supply chain disruption.
“That said, this is a phenomenon that started while the UK was still in the EU so it is hard to put it down entirely to Brexit.”
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Despite Mr Ruparel’s assessment, the Office for Budget Responsibility has predicted Brexit will eventually cause a 15 percent drop in the value of trade in the UK’s economy.
Mr Ruparel said: “It is easy to reach for [Brexit] as an excuse for all problems but doing so risks obscuring the other complex trade challenges business are facing and makes it harder to find the right solutions.”
Since leaving the EU’s orbit, the UK has also signed several bespoke free trade agreements, including with Australia, New Zealand and Japan.
However, Adam Posen, a former policymaker at the Bank of England, suggested there was “no chance” of a similar accord being signed between the UK and the US.