EU leaders failed late Tuesday to finalize the vaccine passport scheme that holiday hubs see as a vital lifeline for the coming peak tourist season.
Negotiators from the European Parliament, the European Commission and member state governments would resume talks on Thursday, the Portuguese EU presidency said.
The digital certificate could open up more EU countries to travel within the bloc as soon as late June, even as the coronavirus pandemic continues and concern about virus variants remains high.
The document is to indicate whether a person has tested negative for Covid-19, has recently recovered from the virus that causes the disease, or has been vaccinated.
This could provide the basis for waiving the quarantine requirements currently in place for many trips within the bloc. However, it remains up to each of the 27 member states to decide what kind of perks the certificate brings within its borders.
German MEP Cornelia Ernst said the negotiators failed to agree on the core issues.
Discussions still revolved around key points such as the EU Parliament's demand for access to free tests for everyone, Ernst said after the day's negotiations.
Green certificate
The European lawmakers also refused to allow individual EU countries to be given the opportunity to impose their own entry restrictions on arrivals from other EU nations even if they have a certificate.
The EU executive branch set out its proposal for a "digital green certificate" in March, kicking off negotiations.
Justice Commissioner Didier Reynders has previously stressed that the issue of free testing is up to national governments.
Greece has already relaxed entry requirements for foreign visitors who have been jabbed, and Spain allows EU tourists in on condition of a negative test.
One sixth of the European Union's adult population is now fully vaccinated, according to the European Centre for Disease Prevention and Control. The commission's target is to raise this to 70% by September.
