I would add that at least some of the weakness in both imports and exports, even that part which is due to Brexit, should be temporary. Businesses are still adjusting to the new rules and systems.

There are still plenty of unknowns here, including those around the Northern Ireland Protocol, and the repeated delays on UK border checks. Just as Brexit uncertainties have held back investment, it is also likely that they have held back both imports and exports.

In the meantime, the impact on the economy is still uncertain, too. Some of the apparent falls in trade may simply reflect differences in the ways that the same flows are recorded before and after Brexit, and therefore have no real impact at all.

The economic impact of a matching reduction in the volumes of both exports and imports is also debatable. (The UK trade deficit has recently widened in value terms, but this partly reflects the higher cost of imported commodities due to global events.)

Even if the OBR’s 15pc figure for the long-term hit to trade turns out to be correct, there is still a big leap between this and a 4pc fall in GDP. To get to this number you have to make a strong assumption about the link between trade openness and productivity.

Again, it is reasonable to assume that the disruption to trade has a negative impact on productivity (otherwise, why was trade happening in the first place). But the hard evidence for the magnitude of this effect is still pretty thin, especially for economies like the UK that are already relatively open and developed.

Even the historical studies typically relied on by Brexit sceptics show results that are imprecise and only marginally significant in statistical terms. Other studies have found no link at all, or that productivity appears to drive trade, rather than the other way around.

Productivity is notoriously hard to explain or predict – as the OBR itself has repeatedly found. While it is plausible that a reduction in trade intensity alone could reduce productivity, Brexit might have some positive effects on productivity too. For example, better management of immigration could help to shake the UK out of a low pay and low productivity trap.

Finally, none of this takes account of any free trade deals that have yet to be agreed, or any regulatory changes that might happen in future. This is because OBR forecasts are always based on current government policies. Of course, some would assert that the economic benefits here will be small. But that’s only an assumption, too.

What this also shows, however, is the importance of making the most of the opportunities that Brexit does create. If the only substantial change is that it is more expensive to trade with the rest of Europe, UK consumers will be worse off. Probably as not as much as some think, or would have us believe, but a reduction in trade would still be a “bad thing”.

Vague appeals to the benefits of import substitution, including greater self-sufficiency and a boost to domestic manufacturing, should cut no ice here. Instead, Global Britain must embrace freer trade with the rest of the world, as well as smarter regulation at home. This is what Brexit should be about.

Julian Jessop is an independent economist. He tweets @julianhjessop