CALAIS, FRANCE (REUTERS) - Red tape, new "smart borders" that don't talk to one another and ill-prepared traders threaten border chaos, French truckers warned a day before Britain restores a hard frontier with Europe.

After clinching a narrow trade deal a week ago, Britain will complete its transition out of Europe's single market at 2300 GMT on Thursday (Dec 31).

While the trade agreement maintains zero-tariff and zero-quota access between Britain and its biggest trading partner, there will be new customs formalities and paperwork.

Disruption is inevitable, said Sebastien Rivera, head of the National Federation of Road Hauliers in the northern Hauts-de-France region, home to the port of Calais and Eurotunnel terminal through which millions of trucks pass each year.

"If you listen to the French and British authorities, the talk is of smart borders and full readiness, but that doesn't stop us having doubts," Rivera told Reuters in the northern town of Arras.

"It will be a real headache. Some companies will be ready, others won't. It's a major change of habits that lies ahead."

He said it was inevitable that not all businesses would be ready to navigate the raft of paperwork, including customs and safety declarations, and the IT systems necessary to trade between Britain and the EU from Jan 1.

Heavy stockpiling by British companies as the trade negotiations went down to the wire should mean freight volumes in January would be lower in January and allow for the new system to bed down, Rivera said.

"Our worry is what happens when we return to 'normal business' in mid-January," he continued.

Smart border?

Britain formally left the EU on Jan 31, 47 years after joining and over three years after its 2016 referendum on Brexit, but then entered a transition period during which rules on commerce and travel were frozen until the end of 2020.

While Britain will phase in full customs formalities over six months, EU states are imposing them immediately.

French customs have employed an additional 700 agents nationwide to deal with the post-Brexit trade rules - almost half of them will be based at Calais' port and Eurotunnel terminal.

"We're as ready as we can be. Are exporters ready?" said regional custom's chief Gilbert Beltran. "That's where the unknown in this smart border lies."

France and Britain's new smart borders are not synchronised.

Beltran declined to predict when the systems might interconnect.

Under the smart-border system, drivers coming into France who have pre-lodged export requests with the French authorities online will have a barcode scanned in say, Dover, and their number plates automatically read by cameras before a risk assessment is made while they cross the Channel.

During the journey, drivers will receive a message telling them whether they can drive off freely through a "green lane" on arrival on French soil, or if they'll be directed to an "orange lane" for further checks.

Haulage firms fear that if too many trucks are not armed with the correct customs documents by their clients, then the trucks requiring further checks will snarl up the borders.

Calais port and the Eurotunnel terminal has enough parking space that such additional checks could be halted for up to two hours without disrupting the flow of other traffic, Beltran said.

"Above two hours we will start having problems. But that seems a very unlikely scenario," he added.

For decades, the economic fortunes of Calais, from where Britain's southern shores 23 miles away are clearly visible, have been deeply intertwined with those of Britain itself.

Britain's divorce with the European Union marked a sad moment, said Calais port chief Jean-Marc Puissesseau.

"Did Britain ever really integrate in Europe? - I'm not sure. They always had one foot in and one foot out," Puissesseau said. "We have Brexit, and a (trade) deal. It could have been worse."