A British nurse who had Ebola diagnosed after returning from Sierra Leone has been described as being in a critical condition by the London hospital treating her.

The Royal Free Hospital said on Sunday it was “sorry to announce that the condition of Pauline Cafferkey has gradually deteriorated over the past two days”.

Ms Cafferkey, from South Lanarkshire, Scotland, and who worked in Bermuda as a nurse for 17 months from September 2005 to February 2007, was given an experimental antiviral drug and blood from disease survivors.

Meanwhile, a patient in Swindon, England, has tested negative for Ebola. Great Western Hospitals NHS Foundation Trust confirmed it was testing an individual with a history of travel to West Africa as a “precautionary measure”. The Trust added that those using the hospital should not be concerned.

Ms Cafferkey, a public health nurse, learnt she had Ebola last month after volunteering with Save the Children in Sierra Leone.

British Prime Minister David Cameron posted yesterday on Twitter: “My thoughts and prayers are with nurse Pauline Cafferkey who is in a critical condition with Ebola.”

Health Secretary Jeremy Hunt also expressed his concern, adding: “I know Dr Mike Jacobs and his team at the Royal Free Hospital are working tirelessly to provide her with the best possible care.”

Scotland’s First Minister, Nicola Sturgeon, tweeted: “My thoughts are with Pauline and her family at this extremely difficult time. Thanks to all who are caring for her.”

Ms Cafferkey had travelled home via Casablanca, Morocco, and London’s Heathrow.

She was screened for the disease at Heathrow, where she told officials she believed a fever might be developing. Her temperature was taken seven times, six of them within a 30-minute period, and was normal each time, so she was allowed to fly to Scotland.

The government’s chief medical officer, Dame Sally Davies, said the case had raises questions about airport screening procedures.

Ms Cafferkey was later placed in an isolation unit at Glasgow’s Gartnavel Hospital after becoming feverish, before being transferred by RAF Hercules plane to London and on to the Royal Free.

Ms Cafferkey’s is the second British case of Ebola. Another nurse, William Pooley, recovered last September after also being treated at the Royal Free Hospital.

He donated blood plasma and was treated with the antiviral drug ZMapp, of which there are no stocks left.

Microbiologist Professor Hugh Pennington said patients responded to Ebola treatment differently.

“Some patients with Ebola get sick then they get better,” he said. “Not everybody dies.”

For this reason, he added, it was “very difficult” to tell how effective treatments would be — especially when “relatively small numbers of people are being treated with these various experimental approaches”.

David Mabey, an expert in communicable diseases from the London School of Hygiene and Tropical Medicine, also said that Ms Cafferkey’s reaction to the virus would have been hard to predict.

“The critical period is in the first four or five days after it is diagnosed, because if you are going to get worse then that’s when it happens,” he said.

Bermuda has a health screening policy for persons arriving from Ebola-affected countries, with officials at LF Wade International Airport checking their travel histories.