The World Health Organization (WHO) has issued a fresh warning to travelling football fans ahead of the EURO 2020 quarter-finals.
Thousands of supporters are permitted to watch the four knockout games taking place on Friday and Saturday, with St. Petersburg, Munich, Baku, and Rome as the host cities.
"I am not here to pour cold water on any EURO 2020 fan or anyone’s holidays," said Hans Kluge, WHO Europe Regional Director, in a statement.
"But before we watch our players, and before we all pack and go for some well-deserved rest near home or far away, it is my imperative to give [some] messages."
"If you decide to travel and gather, assess the risks and do it safely, keeping all life-saving reflexes of masks and self-protection especially indoors and in crowds."
Germany's Interior Minister has criticised UEFA, the European football governing body, for allowing 40,000 fans to watch England’s European Championship match against Germany at Wembley Stadium on Tuesday.
When asked about the decision, Horst Seehofer said UEFA's position was "absolutely irresponsible".
"I have the suspicion that this is about commerce again, and commerce must not outshine the protection of the population against infection," he added.
The WHO said the number of COVID-19 cases in Europe had risen by 10% in the last week, largely due to an increase in travel and mass gatherings, as well as the easing of restrictions.
A significant concern for European countries remains the Delta variant of the coronavirus, which was first detected in India.
Kluge has warned that the continent will be "delta dominant" by August and that vaccination rates are still too slow.
"By August, the region will not be fully vaccinated," he said in a statement.
"63% of people are still waiting for their first jab, and in August the WHO European Region will still be mostly restriction-free, with increasing travels and gatherings."
"There will be a new wave in the WHO European Region unless we remain disciplined ... and unless we all take the vaccine without hesitating when it is our turn."