With arms flailing and raised voices, a shipping manager and a customs chief in the Belgian port of Zeebrugge talk through all the things that could go wrong if the Brexit talks fail.

Trucks backed up for miles on the highway. Car manufacturing plants idled as deliveries are delayed. Mountains of paperwork paralysing small companies that can't afford teams of lawyers.

Not since the 1970s have port towns like this had to deal with customs in their massive trade with Britain, and that could change on Brexit day on 30 March 2019. Britain and the European Union are struggling to agree on the terms of business after that date, raising the chances of a sudden return of tariffs and border controls. And while Brexit has not so far been a big concern for many people in continental Europe, the prospect of there being no deal is starting to unnerve companies and authorities – particularly in places like ports that operate on the front lines of global trade.

“It is scary,” says Marc Adriansens, the managing director of the ICO car shipping company. “Now, all companies around here and on the continent, they start to be worried. And they say: ‘Okay, it could be serious'.”

Kristian Vanderwaeren, the administrator-general of Belgian customs, says this will cost companies a lot to prepare for. “It's a huge, huge problem,” he says.

Ominous warnings of what would happen if trade is suddenly disrupted between Britain and the EU have dominated headlines since the campaign before the Brexit vote in June 2016.

The aim for both Britain and the EU is to agree on the terms of future trade as well as on two-year transition period after Brexit day that would give companies and governments more time to adapt to any new rules.

The lack of progress is raising the risk of Britain falling out with no agreed terms, a feared situation called variously a “hard”, “no-deal” or “cliff-edge” Brexit.

What is at stake is visible during Mr Vanderwaeren's check of the port – in the brisk traffic of trucks on the aptly-named Britannia dock. Everything from engine parts to orange juice is lined up for transport to Britain, part of the half-a-trillion pounds in goods and services traded between the continent and the UK every year.

Brexit threatens life on the Irish border: in pictures Show all 15 1 /15 Brexit threatens life on the Irish border: in pictures Brexit threatens life on the Irish border: in pictures An abandoned shop is seen in Mullan, Co Monaghan. The building was home to four families who left during the Troubles. The town was largely abandoned after the hard border was put in place during the conflict. Mullan has seen some regeneration in recent years, but faces an uncertain future with Brexit on the horizon Reuters Brexit threatens life on the Irish border: in pictures A defaced ‘Welcome to Northern Ireland’ sign stands on the border in Middletown, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Mervyn Johnson owns a garage in the border town of Pettigo, which straddles the counties of Donegal and Fermanagh. ‘I’ve been here since 1956, it was a bit of a problem for a few years. My premises has been blown up about six or seven times, we just kept building and starting again,’ Johnson said laughing. ‘We just got used to it [the hard border] really but now that it’s gone, we wouldn't like it back again’ Reuters Brexit threatens life on the Irish border: in pictures Farmer Gordon Crockett’s Coshquin farm straddles both Derry/Londonderry in the North and Donegal in the Republic. ‘At the minute there is no real problem, you can cross the border as free as you want. We could cross it six or eight times a day,’ said Crockett. ‘If there was any sort of obstruction it would slow down our work every day’ Reuters Brexit threatens life on the Irish border: in pictures John Murphy flies the European flag outside his home near the border village of Forkhill, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Potter Brenda McGinn stands outside her Mullan, Co Monaghan, studio – the former Jas Boylan shoe factory which was the main employer in the area until it shut down due to the Troubles. ‘When I came back, this would have been somewhere you would have driven through and have been quite sad. It was a decrepit looking village,’ said McGinn, whose Busy Bee Ceramics is one of a handful of enterprises restoring life to the community. ‘Now this is a revitalised, old hidden village’ Reuters Brexit threatens life on the Irish border: in pictures Union Flag colours painted on kerbstones and bus-stops along the border village of Newbuildings, Co Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures Grass reflected in Lattone Lough, which is split by the border between Cavan and Fermanagh, seen from near Ballinacor, Northern Ireland Reuters Brexit threatens life on the Irish border: in pictures Donegalman David McClintock sits in the Border Cafe in the village of Muff, which straddles Donegal and Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures An old Irish phone box stands alongside a bus stop in the border town of Glaslough, Co Monaghan Reuters Brexit threatens life on the Irish border: in pictures Billboards are viewed from inside a disused customs hut in Carrickcarnon, Co Down, on the border with Co Louth in the Republic Reuters Brexit threatens life on the Irish border: in pictures Seamus McQuaid takes packages that locals on the Irish side of the border have delivered to his business, McQuaid Auto-Parts, to save money on postal fees, near the Co Fermanagh village of Newtownbutler. ‘I live in the south but the business is in the North,’ said McQaid. "I wholesale into the Republic of Ireland so if there’s duty, I’ll have to set up a company 200 yards up the road to sell to my customers. I’ll have to bring the same product in through Dublin instead of Belfast’ Reuters Brexit threatens life on the Irish border: in pictures A disused Great Northern Railway line and station that was for customs and excise on the border town of Glenfarne, Co Leitrim Reuters Brexit threatens life on the Irish border: in pictures Alice Mullen, from Monaghan in the Republic of Ireland, does her shopping at a former customs post on the border in Middletown, Co Armagh. ‘I’d be very worried if it was a hard border, I remember when people were divided. I would be very afraid of the threat to the peace process, it was a dreadful time to live through. Even to go to mass on a Sunday, you’d have to go through checkpoints. It is terribly stressful,’ said Mullen. ‘All those barricades and boundaries were pulled down. I see it as a huge big exercise of trust and I do believe everyone breathed a sigh of relief’ Reuters Brexit threatens life on the Irish border: in pictures A bus stop and red post box stand in the border town of Jonesborough, Co Armagh Reuters

Ever since the Middle Ages, ships from England have been mooring on the Flemish coast to unload wool and pick up local cloth and other goods from European partners far and wide.

Commerce has grown over time and Zeebrugge is now the port that handles the most cars anywhere in the world, with about 2.8 million rolling through every year – one million of which are to and from Britain.

The local authorities are getting ready for the worst of Brexit outcomes to protect that business.

Joachim Coens, the CEO of Zeebrugge port, says that if tariffs and customs checks on trade with Britain do in fact return, new digital technologies can be used to smoothen out border controls somewhat.

Mr Vanderwaeren plans to double the number of customs officers in Zeebrugge by hiring another 140 to check the Belgian border. The IT system has been upgraded, scanning machines for customs checks have been bought and more sniffer dogs will be made available.

“We are preparing for a Brexit-proof port,” said Mr Coens.

The Republic of Ireland, an EU member country, gets most of its goods transported through UK territory and has sought to ensure it is not cut off from the rest of the bloc because of Britain's exit.

It got help from a new ship, formally called the “Celine” but also dubbed “the Brexit Buster”. that will be able to transport goods directly between Dublin and Belgium, bypassing Britain.

Irish taoiseach Leo Varadkar was on hand when the ship, which is 234 meters long and has nine decks big enough to carry vehicles, was christened in April.

That will not be enough, however, to soften the impact of a “hard” Brexit on Zeebrugge, where nearly half of all goods shipped through the port either goes to or comes from Britain.

Mr Adriansens says his company handles half of the one million cars traded annually with Britain through the port.

After the Brexit vote in 2016, sales went down by 15 per cent and have not recovered. He says there could be another 20 per cent drop if Brexit happens without a trade deal.

“Out of the 500,000, this is 100,000 cars...This is something we don't like to lose, but we cannot control that,” he said.

Beyond the volume of business, the speed of transport is also key, not just for perishable goods like food that can rot if left in containers but also for manufacturing products.

A truck is checked by customs officers at a checkpoint in Zeebrugge (AP)

Modern car plants, for example, do not store many parts on location, meaning their production schedule depends entirely on the timely delivery of the parts, often from various countries.

That production could get held up if Brexit creates traffic jams on both sides of the channel.

About 4,000 trucks drive through Zeebrugge to Britain, says Mr Coens.

“If they are even blocked for a few hours then you have 60 kilometres (40 miles) of blockage,” Mr Coens added. “Nobody wants that.”