Miguel Pajares is being monitored in an isolation ward at the Hospitaller Order of Saint John of God hospital in the Liberian capital of Monrovia.
The condition of the priest, who also suffers from a heart condition, was continuing to worsen on Monday morning, Spanish national daily El Mundo reported.
Pajares visited the hospital only recently to visit the clinic's director Patrick Nshamdze, who has since died from the deadly disease.
The Spanish priest and five other men are currently being treated for fever and other Ebola-like symptoms with paracetamol and saline solution.
However, sources at the hospital said they were yet to carry out definitive tests confirming Pajares is suffering from the Ebola virus.
They were hoping to be able to do so, but conceded that Liberian authorities were overwhelmed in the face of the outbreak.
The Hospitaller Order of Saint John of God hospital has been closed to all patients expect for the six men who remain in isolation, with all other suspected cases of Ebola now being treated at the city's Elwa hospital.
On Sunday, Spain's foreign ministry warned nationals against visiting countries in West Africa affected by the current Ebola outbreak.
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The US is stepping up efforts to fight an outbreak which has claimed more than 800 lives and which US health authorities describe as "out of control".
Ebola, which spreads through contact with blood and bodily fluids, has a mortality rate of 50 to 90 percent. No specific treatment regime exists.