The previous World Cup selection process, held in 2010, was so tarnished with allegations of vote-trading that FIFA was forced to change the procedure. For the first time, its entire membership will get to vote on the host; in the past, that choice had been reserved for its 24-member executive board. More than the half the members of that group in 2010 were later implicated in the wrongdoing during the process, which led to Russia’s hosting the 2018 tournament, and Qatar’s victory in the bidding for the 2022 event.

North American soccer leaders had hoped to avoid a bidding competition altogether by lobbying to expedite the selection process, but FIFA last year rejected a request to review the North American bid — widely seen as the heavy favorite to win — before opening up the competition. That opened a pathway for Morocco to enter the race for the fifth time. The North African country lost to a United States bid in 1994 and also failed to secure the event in 1998, 2006 and 2010. Details produced by American investigators in a 2015 corruption indictment revealed Morocco had tried to bribe voters in two of those races.

Several African soccer leaders, including the regional soccer president Ahmad Ahmad, have given their public backing to Morocco’s bid for 2026. The 54-member continental bloc is expected to vote next week on whether to endorse the bid. Such a move also could violate FIFA guidelines, according to Ms. Samoura’s letter.

“All members of the FIFA Council and delegates of the FIFA Congress should take their personal shortlisting or selection decision on the basis of their own assessment of the merits of the bids without being influenced by supportive or adverse comments of other officials,” she wrote. “Therefore, all officials are requested to refrain from expressing publicly their personal opinion about the merits of one or more bids during the bidding procedure.”

The choice of World Cup host is critical to FIFA’s finances. The organization has been bleeding cash as a result of the United States corruption inquiry and related legal matters. FIFA’s president, Gianni Infantino, is also trying to make good on a 2016 campaign pledge to quadruple development funding to soccer’s 211 member associations.

An expanded World Cup in North America would smash revenue and attendance records. The 1994 tournament sold more tickets than any other before or since, even though it involved only 24 teams, and thus had fewer matches.