Spain star Bonmati in hospital with viral meningitis
Bonmati missed Friday's friendly win over Japan in the world champions' final warm-up match before the start of Euro 2025 next week.
The RFEF said in a statement that Bonmati was "under ongoing medical observation" after tests led to her being diagnosed with viral meningitis.
Bonmati, 27, did not train on Thursday after feeling unwell and later posted a photo of herself in a hospital bed watching the 3-1 victory over Japan.
Spain coach Montse Tome said after the match she was unsure if Bonmati would recover in time to play at the European Championship, which runs from July 2-27 in Switzerland.
"Talking about meningitis can be scary but in theory it is under control," said Tome.
"We don't know long she's going to be absent. I can't say anything more than that.
"Aitana is an extremely important player, so we're going to wait for her as long as possible."
Meningitis is an infection of the protective membranes that surround the brain and spinal cord. Viral meningitis is less serious than the rarer bacterial variant, which can lead to death within 24 hours.
Spain begin their Euro 2025 campaign against neighbours Portugal on July 3 in Bern. They are also drawn with Belgium and Italy in Group C.