LONDON — A British health worker who is being treated for Ebola in a London hospital is now in critical condition, her doctors said on Saturday.

The patient, Pauline Cafferkey, a nurse from Scotland who had volunteered with the charity Save the Children to care for Ebola victims in Sierra Leone, returned to Glasgow, Scotland, last Sunday. She was hospitalized with a fever and given an Ebola diagnosis on Monday, then transferred to an isolation unit at the Royal Free Hospital in London.

Initial reports suggested that Ms. Cafferkey’s condition was stable, but the hospital said in a statement released Saturday that it was “sorry to announce that the condition of Pauline Cafferkey has gradually deteriorated over the past two days.”

The case of Ms. Cafferkey, who had flown via Casablanca, Morocco, and London Heathrow Airport to Glasgow, has raised questions about the effectiveness of the Ebola screening at Heathrow, a global air hub. The nurse had told officials at the airport that she believed she was developing a fever, and her temperature was taken seven times over a short period, the BBC reported. But when it was normal, she was allowed to continue her travel.