Epidemiologist Maria Van Kerkhove, WHO’s Covid-19 technical lead, said watching the 60,000 fans packed into the stadium was like “watching transmission happening in front of my eyes.”
She said the virus pandemic was not taking a break just because it was the Euro finale.
Her comments came hours before England’s 55-year wait for a major win ended in defeat and Italy took home the trophy.
In the tweet rant, Miss Kerkhove added: “#SARSCoV2#DeltaVariant will take advantage of unvaccinated people, in crowded settings, unmasked, screaming/shouting/singing. Devastating.”
There were also reports of some ticketless people being inside the stadium for the whole game and of genuine ticket-holders being afraid to confront those occupying their seats.
It comes after the Prime Minister Boris Johnson defended his decision to allow more than 60,000 people to attend the final earlier this month.
He had said it was being hosted in a “careful and controlled manner with testing of everybody who goes there”.
Mr Johnson also previously said that the vaccine rollout has a “considerable wall of immunity”.
While the country has administrated more than 80 million doses, infection rates in England have climbed to their highest level in five months - according to Office for National Statistics estimates.