Scotland, Wales and the Republic of Ireland are considering launching a joint bid to stage the 2020 European Championship.
The deadline for initial written expressions of interest is under 24 hours away, but the duo are believed to be ready to rival Turkey's bid.
Only three years ago, Scotland and Wales pulled out of the bidding for Euro 2016, with France now set to host the quadrennial tournament.
Secret discussions have been held between both parties for several weeks, according to insidethegames, and a formal application must be submitted to UEFA before Tuesday's midnight deadline.
Only Turkey has formally applied to host the Euros in eight years' time, and if they were to remain the only contender, UEFA has conceded the country would almost certainly be awarded the event by default.
However their bid conflicts with Istanbul's for the summer Olympics and Paralympics the same year.
UEFA President Michel Platini and International Olympic Committee chief Jacques Rogge have both clarified both events cannot be staged in the country.
Baku, Doha, Madrid and Tokyo are the other city candidates to host the 2020 Games.