Bidders must choose ``one or the other'' option to target by a March deadline, UEFA president Aleksander Ceferin said at a news conference, adding ``it's not for both.''
The Euro 2028 bid timeline was announced two months ago with a host set to be chosen in September 2023.
Italy, Russia and Turkey were widely expected to enter the Euro 2028 contest to host the tournament alone.
Adding the 2032 tournament could tempt in a country or joint bid which currently wants to be Europe's preferred candidate in FIFA's contest to choose a 2030 World Cup host.
A Spain-Portugal bid and a combined British and Irish candidacy have long been seen as UEFA's best options. Ceferin has repeatedly said Europe will propose only one to FIFA to avoid splitting its support.
The size of future Euros tournaments is unclear. UEFA said in October the current 24-team format was only provisional for future bids and could change.
The bid timetable could also change, Ceferin said Thursday, according to how many of UEFA's 55 member federations show their interest in March.
UEFA has sped up its host selection process as FIFA pushes to double the number of men's World Cups to every two years instead of four.
European and South American soccer leaders have strongly opposed FIFA's idea and warned of a boycott by their teams.
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