The Mexican unit, known as Banamex, saw an 11% drop year-on-year in December 2022 in its deposit accounts, which does not include its pension unit, according to the latest data from the bank regulator CNBV.

The company's customer numbers for general deposits fell to 8 million from nearly 9 million between December 2021 and December 2022, official data shows.

Its loan book also saw below-average growth during the period in dollar terms, the regulator's data shows, while the raw number of credit accounts fell 14% to 1.2 million from 1.4 million.

Banamex also ranked third among the five top banks for growth in deposit volumes, measured in pesos, during the period.

The bank in a statement said that "the amount of total client (deposit) resources managed by Citibanamex grew in December 2022 compared to December 2021." They declined to comment on account numbers.

Citi's year-long journey to sell its retail unit remains unfinished. Grupo Mexico, headed by billionaire German Larrea, is among the final candidates, Reuters reported.

It is unclear what impact - if any - the sale announcement in January 2022 has had on customer appetite.

Still, four of the other largest banks in Mexico - Banorte, Santander, BBVA Mexico, and HSBC - saw customer deposit account numbers grow in the period, official data shows.

They also all saw larger loan-book growth than Banamex in dollar terms, and all saw an increase in the raw number of credit accounts. Banorte and HSBC declined to comment, while BBVA did not respond to a request for comment.

Santander said they had pursued an "aggressive" strategy to grow the bank.

Carlos Alberto Gonzalez Taberes, an analyst at Monex, says that at face value, the sale process appears to be driving the drop in customers.

"This was always going to be a risk... when something like this is announced, customers naturally feel uncertainty about what will happen with their accounts, and they choose to move," he said.

Arturo Morales Castro, a finance professor at the National Autonomous University of Mexico who has followed the deal, agreed.

"The data reflects the uncertainty from the sale of Citi... There has been a migration of clients to other banks," he said.

While deposit volumes and the number of users are not the primary metrics for banks' profitability, Morales flagged they were nonetheless important indicators.

Meanwhile, banks like mid-sized lender Banregio have said in earnings calls that they see the sale process as an opportunity to win clients from Banamex.

The potential exodus in customers could be among the challenges facing Banamex's buyer, including a limit on cutting costs given that Mexican President Andres Manuel Lopez Obrador has said he opposes sweeping layoffs.

One area, however, where Citibanamex did not see a drop was in its pension business, which forms part of the sale, too.

Citi's take in the Afores market went from 9.6 million accounts in December 2021 to 9.9 million the following year, according to the latest government data.

Only a handful of banks partake in Mexico's pension offering, making the Afore unit an important part of Banamex's valuation.

Banamex is publishing its results on Monday, including its internal metrics for client acquisition.

(Reporting by Isabel Woodford; Editing by Nick Zieminski)

By Isabel Woodford