Two cross-channel operators say UK citizens cannot travel through France to their homes in the EU.
Two cross-channel operators say French Covid rules have been tightened meaning British people can no longer travel through France by road to reach their homes within the EU.
P&O Ferries and Eurotunnel say changes were made by the French on Tuesday.
But the French Interior Minister told news agency AFP there were no new rules and it was only applying restrictions introduced before Christmas.
The BBC has been told the UK government is urgently seeking clarification.
The UK Foreign Office has updated its travel advice for France to state: “The French government have indicated that UK nationals travelling from the UK who are not resident in France will not be permitted to transit France to return to their country of residence unless they are travelling by air.”
Earlier this month, French ministers tightened travel rules for UK travellers, citing concerns about the spread of the Omicron variant.
Since 18 December, British citizens have needed an essential reason to travel to France, with some exemptions in place.
However, on Wednesday, the Eurotunnel shuttle said British people “can no longer transit France by road to reach their country of residence in the EU” unless they hold French residency.
The firm added that it is “unable to answer individual questions regarding the new requirements”.
P&O Ferries has now made a similar announcement, while fellow ferry company DFDS said it was “waiting for confirmation from the Department for Transport and will keep its passengers informed through the usual channels”.
The rail passenger carrier Eurostar said it was not aware of any further changes that affect its customers.
It is not clear how many people will be affected by this latest apparent tightening of the rules.
The British Embassy in Belgium has tweeted that it is aware that “some British nationals seeking to return home to Belgium from the UK by road via France are experiencing difficulties” and it is “urgently seeking clarifications from the French government”.
Speaking to AFP, the French Interior Ministry said that following Brexit, British people were “third-country nationals” and that it therefore “seems logical” that travellers from the UK should not be allowed to “transit to another EU country”.
The French Embassy in London told the BBC the only “authoritative source” was its own website and that of the French Interior Ministry.
The French government website states that both vaccinated and non-vaccinated people must have a compelling reason to travel between the UK and France. It also sets out the circumstances under which travel would be allowed – however people are specifically banned from travelling as tourists or for professional reasons.
In a statement, P&O said: “The decision on whether or not there is a compelling reason for travel sits with French Border Police on a case by case basis.”
It warned that restrictions had been “subject to change at little notice” and advised customers to check the French and UK foreign offices for information.
The confusion over travel rules comes as France is experiencing record numbers of infections with over 200,000 positive cases reported on Wednesday.
On the same day, the UK also saw cases jump to a record high of 183,000 people testing positive.