She was placed in isolation at Gartnavel Hospital in Glasgow before being transferred to the Royal Free Hospital in north London, where the UK’s specialist Ebola unit is based.
The hospital said in a brief statement on Saturday: “The Royal Free London NHS Foundation Trust is sorry to announce that the condition of Pauline Cafferkey has gradually deteriorated over the past two days and is now critical.”
The 39-year-old’s sudden change in condition came after her doctor described her as sitting up, eating, drinking and communicating with her family on new year’s day.
Dr Michael Jacobs warned that she faced a “critical” few days while she was treated with blood from a survivor and an experimental antiviral drug which is “not proven to work”.
David Cameron said on Sunday that Ebola was the “uppermost thing” on his mind following the news of Ms Cafferkey’s condition.
The Scottish nurse is the first person to be diagnosed with Ebola within the UK.
Meanwhile tests on a patient with a history of travel to west Africa have come back negative for Ebola. The patient was being kept in isolation at Great Western Hospital at Swindon as tests were carried out as a “precautionary measure”.
A spokesman for the hospital said: “The test results have come back negative. The patient is continuing to stay within the hospital for treatment.”