Belgium’s customs authority is advising companies that export to the UK to halt shipments after Brexit day to avoid customs chaos in the event of a no-deal scenario.

Kristian Vanderwaeren, chief executive of Belgian customs, called for a “Brexitpauze” after 29 March and said firms should do as much of their exporting as they can before new controls have to come in.

“Who are we as customs to give the business world instructions? But we are still asking the SMEs and all other parties to wait. Do the necessary export to your customers before 29 March,” he told Belgian business newspaper De Tijd.

Mr Vanderwaeren said larger industries “such as pharmaceutical companies and car manufacturers” had been “storing stock in the UK for months” to avoid having to get parts through after Brexit but that the vast majority of businesses were not well prepared with just weeks to go.

Belgium’s main port of Zeebrugge is expected to have to deal with around a million additional import declarations and 4.5 million extra export declarations once the UK leaves the single market and customs union.

The warning follows chaos and hours of delays at Calais and the Eurostar terminal in Paris after French customs officers carried out a trial of the sort of checks they would have to impose under a no-deal Brexit.

The customs chief also warned that many small businesses that dealt with the UK were simply not prepared to export and that it would be better for them to pause operations.

“Our customs authority has written letters to some 20,000 companies that trade with the UK telling them they have to apply for an EORI number, which is necessary to be able to import and export,” Mr Vanderwaeren told the newspaper.

“But today, three weeks before the Brexit, customs received only 4,700 positive answers. Customs has even set up a call centre that answers questions and calls companies to get them to register for customs licences.”

France, the Netherlands and Belgium have all hired hundreds of extra customs officers to deal with the potential disruption caused by the UK’s decision.

Unless Article 50 is extended or revoked, Britain is set to leave on 29 March. If Theresa May’s Brexit deal is approved by MPs, the UK will enter a transition period and little will change immediately – but there are few signs the government has the numbers in the House of Commons to get it approved.

If there is no extension of revocation and no transition period, the UK will crash out and the expected chaos at ports and airports will materialise.

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The UK government unveiled plans last month to wave through EU imports under a no-deal Brexit in a bid to head off disruption. Companies bringing products to the UK will not have to make a full customs declaration at the border and will be able to postpone paying import duties.