Some of the cities hosting Euro 2020 matches this summer could be dropped if they cannot guarantee supporters will be allowed into games, says UEFA President Aleksander Ceferin.
The governing body intends to hold the tournament - which has been postponed a year due to coronavirus - across 12 European cities.
However, the varying levels of the virus and the different restrictions in each country mean it is unclear whether fans will be able to attend games in all 12 cities when the tournament kicks off in June.
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Football is still being played in empty stadiums across Europe, but speaking on Sunday, Ceferin said: "We have several scenarios, but the one guarantee we can make is that the option of playing any Euro 2020 match in an empty stadium is off the table.
"Every host must guarantee there will be fans at their games."
UEFA has also denied reports it is going to delay its decision on which of the 12 cities will be retained as hosts, saying the deadline for cities to submit plans over the number of fans they will allow in stadiums remains April 7.
UEFA's executive committee will then meet on April 19, before a UEFA Congress on April 20, where a final decision on host cities must be made.
Ceferin said: "We have set a deadline of April 20 for the final decision on the Euros.
"The ideal scenario is to play the tournament in the original 12 venues, but if that is not possible then it will go ahead in either 10 or 11 countries if one or more of the venues cannot meet the required conditions."
Reports earlier this month suggested Glasgow and Dublin were set to be dropped as two of the 12 host cities over concerns the Scottish and Irish Governments would not permit fans to attend games.
The Scottish Government responded by saying it was continuing its plans to host matches in Glasgow.
Prime Minister Boris Johnson has previously said that the UK is ready to host additional matches, should any host cities be forced to drop out.
However, the Football Association said it had no plans to add additional matches to the seven that are set to be held at Wembley.
Who has made it to Euro 2020 and how will the tournament work?
Coronavirus has put Euro 2020 back a year to 2021, but 24 sides will still play across 12 host cities in a festival of football.
The top two teams from each of the 10 groups qualified for Euro 2020 and the four remaining positions were settled in the play-offs, with Scotland beating Serbia in a penalty shootout to join England and Wales at the tournament.
When and where is Euro 2020?
The 16th UEFA European Championship runs from June 11 to July 11, 2021, and to celebrate the tournament's 60th birthday, 12 cities across the continent have been selected as hosts. The 12 cities and stadiums are:
- Amsterdam (Netherlands) - Johan Cruyff Arena
- Baku (Azerbaijan) - Olympic Stadium
- Bilbao (Spain) - San Mames
- Bucharest (Romania) - Arena Nationala
- Budapest (Hungary) - Puskas Arena
- Copenhagen (Denmark) - Parken Stadium
- Dublin (Republic of Ireland) - Aviva Stadium
- Glasgow (Scotland) - Hampden Park
- London (England) - Wembley Stadium
- Munich (Germany) - Allianz Arena
- Rome (Italy) - Stadio Olimpico
- Saint Petersburg (Russia) - Krestovsky Stadium
Rome's Stadio Olimpico will host the opening match on June 11, while England's national stadium Wembley will stage both semi-finals and the final.