The bottom line is that if Britain were to leave the EU without a trade deal but continue to export the same products there, the customs bill would be in the region of $7.6bn. At today's exchange rate, that's £6.1bn. For comparison, Britain's net contributions to the EU are about £8.5bn.

The highest customs bill in the stone and ceramics sector will come for fine china tableware, such as that made in the potteries of Stoke-on-Trent and surrounding towns. The area voted strongly to leave the EU.

The customs bill solely for non-knitted clothing exported to Italy will be $30.8m annually. British firms who rely on their exports to Italy to stay in business will be hoping that their Italian counterparts will exert pressure in any negotiations. Britain imports 2.5 times as much “non-knitted clothing” from Italy as it exports there, so an unnegotiated Brexit would be costly for Italy too.

Because of high tariffs on the sector, Britain’s clothing and textile industry faces a customs bill of over $1bn dollars. The tariff barriers for the UK’s chemicals and machinery exports are lower, even though those sectors each export three times as much by value, than the fashion industry.

Tariffs on food are some of the highest, and also the most complex; they are typically charged partly on value, partly on weight, and vary by factors such as fat content, the size of the packaging and even the season. Particularly hard-hit products will be yoghurt, beef and pork. For farmers, this unwelcome change will come alongside uncertainty about the future of agricultural subsidies as Britain leaves the Common Agricultural Policy.

In contrast, the trade with France in aircraft parts may be harder to replace. The Airbus factory in Wales, which makes wings for assembly in Toulouse, is one of few in the world.

The British and German car industries are intricately entwined, with parts made in one country being assembled into components in the other. Tariffs on cars and car parts sold to Germany alone would amount to $318.4m annually. But it is possible to overstate German reliance on British participation in its supply chains. Consider that in 2015, Germany imported more brake parts from the Czech Republic, Italy, Poland, France, Spain and China than it did from the UK.

Britain's chemical industry is big and diverse, and its various products face different tariff barriers. For instance, exporters of retail-packed medicines, a big trade volume, could go on selling to the EU without any new charges.

We calculated the cost in customs duties for each product if Britain carried on exporting to the EU as in 2015 but without a deal. The total bill is $7.6bn , but the pain is not felt equally by all industries, nor by exporters to all destinations.

What we did

We took Britain's exports to the EU27 for 2015, as declared to the UN Comtrade database. That year is the latest available data as well as the last full year before the Brexit vote affected the value of the pound. Trade lines are given at varying levels of resolution, the most detailed of which use the six-digit nomenclature of the "Harmonized System". For example, the six-digit classification distinguishes between “prepared cereals” – both roasted and non-roasted – but does not differ between those made from maize or other cereals.

Then, from the World Trade Organisation, we took the EU's declared “most favoured nation”, ie non-punitive, ad valorem tariff rates and applied them to the dollar value of British exports. That means we took the basic EU tariffs that apply to the value of goods imported from WTO members and calculated the hypothetical tariffs payable, at 2015 rates, on British goods imported by the EU in 2015. The tariffs given to the WTO for this purpose are expressed at the same six-digit level in which the trade is declared, but because multiple different tariffs may apply at this level, the number used is an average. For instance, chewing gum containing more than 60% sucrose has a different tariff to chewing gum that contains less than 60% sucrose, but since the UN Comtrade data only goes to the six-digit level, we used an average of the two in our calculations. We don't know if the actual volume of trade tends more toward one or the other.

A further shortcoming of the approach is that it uses only tariffs that apply to the value of goods, not those that apply to goods by weight or unit count, for instance. Happily, the World Bank has prepared a set of “ad valorem equivalents” for certain products. These are rules of thumb allowing us to calculate tariff costs on the dollar value of those trade lines, even though the actual tariffs are applied differently. Here again, we averaged up to the six-digit level.

For anyone still reading, the total customs charge we calculate is $7,606,789,218. (Without the inclusion of the World Bank 'ad valorem equivalents', the sum is $6,712m.) At the 2015 pound-dollar exchange rate given by the bank of England (1.5286) that is £4.976bn annually or £95m per week. At the latest average monthly rate (1.2351) that is £6.158bn or £118m per week.