Champions of Andorra and Montenegro respectively, the clubs will take part in a four-team playoff, hosted at Kopavogsvollur Stadium in Iceland.
Match preview
Ultimately aiming to progress to the first qualifying round proper, Atletic d'Escaldes and Buducnost meet in the semi-finals of the Champions League preliminaries this week, before the final is played out on Friday.
In a process also including San Marino's Tre Penne and playoff hosts Breidablik, the winner of that decider will advance to the first round, where established names such as Ludogorets and Ferencvaros await.
The three teams to drop out will instead be transferred to the Europa Conference League's second qualifying round, so there is plenty at stake when the mini-tournament gets under way in Kopavogur.
Andorran champions Atletic topped the Primera Divisio last season, only four years after they made it back to the top flight by winning the 2018-19 Segona Divisio. Now, they embark on an improbable quest to progress in Europe.
Pipping local rivals Inter d'Escalde to the title by two points, the side managed by former Barcelona youth-teamer and Swansea City left-back Fede Bessone won six of their last eight league matches to secure the title, but now an even greater test lies in store.
By contrast with their unseeded opponents, Montenegrin champions Buducnost have become regulars in UEFA competition since independence from Serbia, and they are setting off on a third Champions League journey in four years.
In all, they have won six national titles and are Montenegro's top club in terms of continental qualification - they have played more seasons and matches in European competition than any other team from the Balkan nation, winning the Intertoto Cup in 1981.
Dethroning previous First League winners Sutjeska - also the Podgorica club's fiercest domestic rivals - was no easy task last term, and after finishing level on 70 points, the pair were only separated by Buducnost's better head-to-head record.
They will now expect to overcome one of Europe's minnows in Tuesday's Champions League opener, and should Miodrag Dzudovic's men progress to the decider, a chance for revenge could arise: Buducnost lost to Breidablik - their fellow seeds in these playoffs - at the Europa Conference League's second qualifying stage last July.
Atletic Club d'Escaldes form (all competitions):
D
L
Buducnost Podgorica form (all competitions):
W
D
L
W
Team News
With the stakes high even at the outset, given the single-leg knockout nature of the preliminaries, Atletic will turn to last term's top scorer Guillaume Lopez for inspiration up front, after the French striker finished their title-winning campaign with 15 league goals.
Alex Martinez - an Andorra international and beach football player - will provide support, having appeared as a substitute in his country's 2-1 Euro 2024 qualifying defeat to Israel last week.
Buducnost, meanwhile, shared the goalscoring burden around more evenly in securing their latest First League triumph, with leading marksman Zoran Petrovic providing seven goals en route to the title.
Goalkeeper Milos Dragojevic and defender Marko Simic did not make the cut for Montenegro's recent qualifiers, but they are two of very few domestic players to feature for the national team over the past few years.
Atletic Club d'Escaldes possible starting lineup:
Lopez; San Nicolas, Sanchez, Alonso, Cistero; Bolivar, Perez, Aguero; Martinez, Lopez, Casadesus
Buducnost Podgorica possible starting lineup:
Dragojevic; Ignjatovic, Djurickovic, Novovic, Simic; Terzic, Mirkovic, Sekulovic; Grbic, Sekulic, Petrovic