N EARLY 3,400 lorries are ferried between Rotterdam’s port and Britain every day. They roll on and off the boats, carrying much of the 54m tonnes of goods that are traded between the Netherlands and Britain each year. The precise form of Brexit is still being wrangled over in Westminster. But unless Britain decides to stay in the EU ’s single market and customs union (which seems unlikely), trade will become less seamless.

Truckers might need to ensure that their cargo has the right paperwork, or risk being turned away at the port. Some British products—such as livestock—might need to be inspected by vets upon entering the EU . Delays at the border could ruin produce going the other way: around €825m ($930m)-worth of flowers and plants are shipped to Britain from the Netherlands every year, says Matthijs Mesken of VGB , which represents Dutch wholesalers and exporters. Delivery takes place a matter of hours after an order is placed.

Brexit contributes to the “pervasive uncertainty” which, in the words of Mario Draghi, the head of the European Central Bank (ECB) , clouds the euro zone’s economic horizon. A study in 2018 by the IMF found that Britain’s economy, already deeply integrated with the rest of the EU , became even more so after the 2007-08 financial crisis. The worst effects of loosening those ties will be felt in Britain. But others in Europe will not go unscathed. Small open economies are the most vulnerable.

The countries with the closest trading ties with Britain include Belgium, Ireland and the Netherlands. According to the IMF, Ireland’s exports of goods and services to Britain amounted to 15% of GDP in 2014-16, and those of Belgium and the Netherlands nearly 10%. All three also rely relatively heavily on imports from Britain.

Ireland and the Netherlands are intertwined with Britain in other ways too. Thanks to history and geography, migrant flows between Ireland and Britain are large, equivalent to nearly a fifth of the population of Ireland. (Malta and Cyprus, which also have historical ties to Britain, see large migrant flows, too, relative to their size.) The Netherlands is a big investor in British business: its stock of foreign direct investment in the country was equivalent to an average of almost 80% of Dutch GDP in 2014-16, the highest share in the EU . Irish and Dutch financial firms have relatively high exposures to British borrowers. Luxembourg also has close trade and investment links, according to analysis by S&P Global, a credit-ratings agency. But that may partly reflect the fact that some British firms register there for tax purposes. Larger countries are better insulated, but certain sectors are still exposed. A study by Hans-Ulrich Brautzsch and Oliver Holtemöller for the Halle Institute of Economic Research finds that 15,000 jobs in Germany—around 1% of employment in its car industry—depend either directly or indirectly on exports to Britain. S&P analysts note that Spain is most exposed through its ownership of banks, telecoms and insurance firms in Britain. The IMF reckons that the long-term impact of Brexit on the EU27 as a whole would be modest, provided a deal is struck. But some countries might still suffer. If Britain left the single market and instead signed a free-trade deal with the EU , the fund reckons output in Ireland would eventually fall by 2.5%, and that in Belgium and the Netherlands by 0.5-0.7%. In the event of no deal, the losses would nearly double.

As small members of a monetary union, these countries cannot expect the ECB to come to their rescue with euro-zone-wide monetary loosening. Instead, their governments would probably need to stimulate their economies. Whereas Ireland and the Netherlands have the fiscal space to cushion the blow, Belgium, with a public-debt ratio of around 100% of GDP , has less room to act.