Global health chiefs have warned that thousands of fans travelling to and from European championship football matches could unleash another wave of the coronavirus pandemic.
The stark prediction emerged just before the start of the last eight in the tournament which is taking place across Europe.
Hans Kluge, the regional director for Europe at the World Health Organisation (WHO), said: “There will be a new wave in the European region unless we remain disciplined.”
He said matches could become so-called super-spreader events. "I hope not,” he added. “But this can't be excluded.”
Switzerland, who beat tournament favourites France in the last-16, take on Spain in Saint Petersburg on Friday evening before Belgium face Italy in Munich.
On Saturday in Baku, the Czech Republic play Denmark and Ukraine go up against England in Rome.
Hundreds of cases have been detected among spectators attending Euro matches, including Scots returning from London and Finns going home from Saint Petersburg.
"What we need to look at is around the stadia,” said Catherine Smallwood, senior emergency officer at WHO's European office.
“How are people getting there? Are they travelling in large crowded convoys of buses? Are they taking individual measures when they are doing that?" she added.
London is due to host the semi-finals and final of the tournament next week.
On Friday, the Office for National Statistics in Britain said one person in 260 in England had Covid in the last week.
The figure is a significant rise on the previous week's data of one in 440.
"Should this mixing happen, there will be cases," Smallwood added. "What we know is that in a context of increasing transmission, large mass gatherings can act as amplifiers in terms of transmission."