The World Health Organisation says crowds at stadiums and in pubs and bars in host cities of football's European Championship have driven the current rise in coronavirus infections on the continent.
A 10-week decline in Covid-19 cases across the region has come to an end.
And according to WHO, a new wave of infections is inevitable if football fans and others drop their guard.
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WHO reported on Thursday that the number of new cases rose by 10 per cent last week, driven by mixing of crowds in the host cities of Euro 2020, travel and easing of social restrictions.
"We need to look much beyond just the stadiums themselves," WHO's senior emergency officer Catherine Smallwood said.
"We need to look at how people get there, are they travelling in large crowded convoys of buses?
"And when they leave the stadiums, are they going into crowded bars and pubs to watch the matches?
"It is these small continuous events that are driving the spread of the virus."
Germany's Interior Minister Horst Seehofer called a decision by European football's governing body UEFA to allow big crowds at the event "utterly irresponsible".
UEFA said in a statement that mitigation measures at host venues "are fully aligned with the regulations set out by the competent local public health authorities".
The rise in new Covid-19 cases is happening as the more contagious Delta variant of the virus spreads rapidly across Europe.
Scottish authorities reported on Wednesday that nearly 2000 people who live in Scotland have attended a Euro event while infectious with Covid-19, with many attending their group stage match against England in London on June 18.
The rise in infections has raised concern that a third wave could spread across Europe in the autumn if people do not get vaccinated.
"The concern of an autumn surge is still there, but what we see now is that it might come even earlier," Smallwood said.
Major concerns about crowds at Euro 2020 semi-finals
With vaccination progressing fast across the continent and people eagerly anticipating the holiday season, it is tempting to view large crowds at Euro 2020 matches as a sign of life returning to normal.
But there are particular concerns about upcoming games in London and Saint Petersburg - two cities particularly affected by the Delta variant.
"If we want to spread the Delta variant around Europe then this is the way to do it," epidemiologist Antoine Flahault told AFP.
Friday's quarter-final between Spain and Switzerland will go ahead in Saint Petersburg, despite Russia dealing with a surge in virus cases and record daily deaths.
Both semi-finals and the final will be played in London, with more than 60,000 fans allowed to attend the games at Wembley.
The Delta variant has also been behind a steep upward curve in cases in the United Kingdom, but that did not stop authorities easing restrictions for England's last-16 win over Germany on Tuesday.
Almost 42,000 fans were inside the 90,000-seat Wembley to witness that game and the images of maskless England supporters wildly celebrating victory have caused some consternation.
"It wouldn't have been very difficult to move these matches to cities where the risk is not so great," said Flahault, the director of the Institute of Global Health at the University of Geneva.
But UEFA told AFP "all the remaining matches will go ahead according to the match schedule as planned".
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