Spanish citizens are selected at random to work in polling stations, with local elections due to be held in Madrid on Tuesday May 4.
The newspaper El Mundo first reported on Wednesday that Marcelo -- who obtained Spanish nationality in 2011 -- had been chosen to participate.
Sources confirmed that to ESPN, although they stressed that Real Madrid expected to be able to resolve the issue to ensure the defender's availability for the game.
Under Spanish law, citizens who are selected to take part in the electoral process can appeal, asking to be excused under a set of specific circumstances including professional obligations.
Real Madrid are due to fly to London on May 4 before facing Chelsea at Stamford Bridge a day later, on Wednesday May 5.
The teams drew 1-1 in their semifinal first leg at the Alfredo di Stefano stadium on Tuesday night, with Marcelo providing the cross for Karim Benzema's equaliser after Christian Pulisic had opened the scoring for the visitors.
The Brazilian has lost his place at left-back in the Real Madrid starting XI to Ferland Mendy this season, but Mendy is currently out with a calf injury and it is not clear if he will available for the Champions League second leg.
If Marcelo and the club's appeals for the player to be excused from his elections role are unsuccessful, he could alternatively travel to London separately from his teammates, arriving on the day of the match itself.
In November 2019, Levante goalkeeper Aitor Fernandez was selected for election duty at a polling station on the same day that his team were due to face Athletic Bilbao, although the club was successful in appealing that decision.