An EU-wide Covid travel certificate comes into force on Thursday just in time for Europe's busy summer vacation period, but the more-infectious Delta variant is already threatening to curtail its use.
The EU document, sporting a QR code and available in digital form on smartphones or hard copy, shows whether the bearer is vaccinated with one of the EU's approved jabs (from BioNTech/Pfizer, AstraZeneca, Moderna or Johnson & Johnson), has recovered from an infection, or has a recent negative Covid test.
Under an EU law adopted this month, the certificate does away with the need for quarantines or further testing when travelling between the EU's 27 countries or four associated European nations (Iceland, Norway, Switzerland and Liechtenstein).
Twenty EU countries have already started issuing and accepting it, with the others expected to follow suit shortly.
But a surge in the Delta variant, first detected in India and now rampant in former EU member Britain, could trigger an "emergency brake" provision suspending its acceptance.
With Delta becoming dominant in Portugal, Germany has announced a ban on incoming travellers from there except for its own citizens or residents.
Even they are required to quarantine for two weeks, regardless of vaccination or test status.
Portugal and Spain on Monday abruptly announced entry restrictions for travellers from Britain, with Lisbon requiring them to be fully vaccinated and Madrid demanding proof of vaccination or a negative Covid test.
Britain's Delta problem
A European Commission spokesman said on Monday that Britain "is now working with us" on aligning international Covid travel requirements with the aim of having mutually accepted documents.
But Britain's rise in Delta infections -- giving it an infection rate more than four times that of the EU -- is generating deep concern on the continent.
At an EU summit last week, German Chancellor Angela Merkel criticised southern EU countries for allowing in Britons with few, if any, Covid checks.
The move gives Brits access to the entire passport-free Schengen zone and has brought them in proximity to vacationing Europeans.
Under non-binding EU guidelines, all member states were urged to allow in only fully vaccinated travellers from outside the bloc, or those with urgent reasons to visit.
But Portugal, Spain and Greece initially opted for a laissez-faire attitude, hoping to revive their vital tourist sectors.
The latest guidelines of France's Ministry of Health says it does not subject fully vaccinated people from "green" countries (including from EU member states,) negative PCR test results for those who were not vaccinated.
Stricter regulations are in place for arrivals from risk-zones, but exemptions can be granted, but all travellers need a "certificate of international travel" which they have to sign.
Currently the number of new cases in France has been stable at around 2,000 per day. But health authorities fear the effect of the Delta virus, which is now also spreading in France, may be visible soon.