European politicians on Thursday approved an EU-wide Covid-19 certificate sought by southern member states to help revive summer tourism.
But the move could be undermined by their different vaccination and testing rules.
Then fate of the project, which tourism-dependent countries Spain and Greece hope will help to resuscitate their economies, hinges on further negotiations between EU politicians, executive commission and member states.
It aims to introduce a standard pass for people who have been vaccinated or tested negative for the virus to travel across the 27-nation bloc.
But national differences over recognising negative antibody tests on top of PCR tests, technical difficulties with a single EU gateway, worries about discrimination against those who have not been vaccinated, and privacy issues mean finding common ground might be tricky, diplomats said.
The probable result is a system that would introduce Covid-19 certificates as a minimum common denominator, but leave leeway to each country on what rights they would be willing to grant to holders of the passes.
In voting overwhelmingly for a certificate, the European Parliament said member states should not impose more travel restrictions on those carrying it, and that free testing should be widely available.
But the European Commission has proposed that countries should be able to retain extra requirements, as many member states want.
"The certificate is not about allowing or forbidding travel," commission spokesman Eric Mamer said on Thursday. "It's a system of information on your health status in relation to Covid.
"Member states have the prerogatives, the responsibility when it comes to health safety measures (required for entry). The green digital certificate does not change this."
As the complex EU process grinds on, many countries are putting their own systems in place, increasing the risk of a disjointed set of rules that would fail to ensure uniform travel across the bloc.
Hungary said on Thursday it would pursue bilateral deals on recognising Covid-19 vaccine certificates that would allow citizens to travel among participating countries.
Others, including Estonia, Lithuania, Greece, Spain, Germany and France, are introducing their own solutions to record vaccinations.
Member states will be allowed to keep their own systems in place even after an EU-wide process is in place.
They would be obliged to honour vaccines authorised for the whole bloc by the European Medicines Agency – those from Pfizer-BioNTech, Moderna and Oxford-AstraZeneca.
But each country would decide how to treat people who received Russian or Chinese shots.