Mari arrived in London on Saturday for his medical with the Gunners technical director, Edu Gaspar, after a deal to sign him on loan with a view to a £7.5m transfer was agreed.
The 26-year-old had his medical over the weekend, but a late hitch has seen the transfer stall.
Negotiations are ongoing between the two clubs, but Arsenal are also considering new targets.
Mari was a vital part of Flamengo’s double-winning season in 2019, helping the Brazilian side to the league title and Copa Libertadores triumph.

