Thousands of truck drivers and their freight remain trapped in mass gridlock at the English port of Dover as French authorities demand coronavirus tests before they can enter.
France partially reopened its borders with Britain following a scare over a rapidly spreading new virus variant.
One by one, trucks passed toward ferries and trains that link Britain with France, as authorities checked that drivers had the negative virus tests now required to cross.
On the French side, the vast Calais port - which normally takes in up to 4,000 trucks a day - remained quieter than usual.
Officials warned the backlog could take days to clear. One UK road haulage expert estimated there could be 8,000 to 10,000 trucks caught up in the chaos near Dover.
Dozens of countries around the world began barring people from the UK at the weekend after Prime Minister Boris Johnson said large areas of southern England had to be placed under harsh restrictions to curb a new, more contagious version of the virus whipping around London and England's southeast.
Britain has seen soaring infection rates in recent weeks, with many hospitals nearing their capacities. On Wednesday, the country reported another 744 deaths and a record 39,237 confirmed cases. Christmas gatherings and festive shopping were cancelled for millions at the last minute in a bid to control the spread of the virus.
The announcement of the coronavirus variant added to anxieties at a time when Europe has been hit by soaring new virus infections and deaths. Europe as a whole has recorded over 500,000 virus-related deaths, according to a tally by Johns Hopkins University that experts agree is probably an undercount due to missed cases and other factors.
But France's move raised the most concern, since France is a major conduit for trade and travel between Britain and the continent. The UK relies heavily on cross-Channel commercial links to the continent for food at this time of year, especially fresh fruit and vegetables.
On Thursday, French Ambassador Catherine Colonna said two dozen French firefighters have been sent to Dover, bringing 10,000 coronavirus tests for drivers desperate to get home for Christmas. British Transport Secretary Grant Shapps said British and French authorities have agreed to keep the border between the countries open throughout Christmas to help hauliers and travellers get home.
France defended its handling of the border situation after the EU's transport commissioner issued unusually strong public criticism.
Commissioner Adina Valean, of Romania, tweeted: "I deplore that France went against our recommendations and brought us back to the situation we were in in March when the supply chains were interrupted."
Some European countries relaxed their travel limits on Britain on Wednesday, though many remain in place. China on Thursday became the latest nation to suspend flights to and from the UK