Chelsea and Real Madrid could both be without several stars for their upcoming Champions League ties due to France's new laws.
There is no longer an exemption for unvaccinated athletes to enter France after the country tightened its Covid-19 rules.
Holders Chelsea face French champions Lille in the last 16 of this season's tournament, while La Liga leaders Real have a difficult tie against Ligue 1 hopefuls Paris Saint-Germain.
On Thursday, UEFA confirmed all teams are required "in principle" to abide by the applicable rules in any country.
European football's governing body is yet to publish its updated regulations, which provide "further guidance" for clubs in Chelsea's predicament.
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Chelsea's away trip to Lille is on March 16, while Real's visit to the French capital to face PSG is scheduled for February 15.
It is unclear how many Chelsea and Real Madrid players - if any - are unvaccinated against Covid.
UEFA are unlikely to move the games to a neutral venue and undermine France's Covid rules - as the organisation's president Aleksander Ceferin is a huge advocate for vaccination - although it remains a possibility.
As it stands, all unvaccinated stars will be treated as positive cases and will be barred from travelling to France.
"UEFA is liaising with the relevant stakeholders across Europe ahead of the resumption of UEFA's club competitions in February," read Thursday's statement.
"The specific conditions under which matches are played in each country are subject to the decisions of the competent authorities in that country.
"Each team will in principle be required to comply with the applicable rules in the country where the match takes place.
"But the specific competition Annexes - still under elaboration - which contain special rules due to Covid-19 applicable to the knockout phase, may provide further guidance in this regard."
Speaking last month, Chelsea manager Thomas Tuchel urged supporters and reporters to stop questioning unvaccinated stars.
"I don't want to get involved in pointing fingers and starting the hunt for non vaccinated people," said Tuchel. "This is a choice to make. Leave it there.
"We cannot force people to get vaccinated. And I will not change my opinion on that. And I am not the guy to comment on that, I am not the expert here.
"There are experts in this country, all over the rest of Europe. Ask them and ask me please about football. I will not get involved. I am vaccinated. I took the decision for me. And that's it.
“You know that it's causing an issue. But it's not that we have all unvaccinated infected. We have vaccinated players who are positive.
"I mean you can have an opinion, I can have an opinion but a player can also have an opinion.
"Then there can be regulations around this, like when you arrive to work your employer can create an environment where you are not allowed to be in the building if you are not vaccinated.
"This can happen so you have to adapt to it and you have to live with the consequences."
