Britain faced the first true test of its Brexit preparations on Monday, with thousands of lorries expected to make the Channel crossing for the first time since the country left the EU's single market and customs union.

Britain left the EU's institutions late Thursday, but light holiday season traffic has meant little work for border officials implementing new customs checks.

However, the government expects thousands of lorries to make the crossing on Monday, bringing goods to and from the continent and testing its plans to keep traffic flowing around key ports such as Dover.

The government, which has employed 1,000 new border officials, is phasing in the new customs checks, with full import control checks for all goods not to be implemented until July.

But it is concerned that truckers will turn up at Dover without the proper paperwork, causing delays at the port itself and blocking surrounding roads if they are turned away.

While it believes most large businesses are ready for the new rules, the government is worried that around 50 percent of medium and small firms have not carried out the measures now needed to export goods to Europe.

In its reasonable worst-case scenario, over half of trucks turning up to Dover may not be ready for travel, leading to queues of 7,000 lorries that will be directed towards a disused local airfield.

That could lead to a return to the chaotic scenes witnessed before Christmas, when thousands of trucks were marooned in Britain after France closed the border to all traffic due to surging coronavirus cases.

The UK government is concerned that truckers will turn up at Dover without the proper paperwork, causing delays Photo: AFP / JUSTIN TALLIS

The lorries ended up being stuck for days at the Manston airfield while the drivers were all tested for the virus.

France still requires a negative test for drivers crossing the Channel, adding an extra layer of complexity for officials trying to keep traffic flowing.

The Department for Transport announced on Saturday the creation of 10 new testing sites, with 10 more opening on Sunday.

In a bid to prevent lorries arriving at Dover unprepared, truckers heading across the Channel are required to obtain temporary permits to allow them to drive on the roads of Kent, the county in which the port is situated.

Cameras will identify any trucks without the Kent Access Permit, with the drivers facing a ?300 ($410, 337 euros) penalty, although it appeared not all truckers understood the new rules.

"Some people are worried because of the new documents needed (for UK exports) -- you saw what happened before Christmas," Romanian trucker Alexandru Mareci, one of the first to make the journey after the new rules kicked in, told AFP.

Mareci, who was stuck in Britain for two days during the French border closure, said he had never heard of the Kent Access Permit.

As well as the new customs checks, free movement of over 500 million people between Britain and the 27 EU states ended as the new arrangements took hold.

Britain voted to leave the EU in 2016.

As part of the divorce deal, the UK remained tied to the EU's institutions for a year-long transition period, during which talks took place to secure a deal on the future trading relationship.

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