Mrs Cafferkey, a Scottish public health nurse who had been volunteering in the stricken West African country, was diagnosed with the deadly virus after returning to Glasgow from Sierra Leone via Casablanca in Morocco.
A brief statement on the hospital’s website said: “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.”
Her sudden change in condition comes just days after her doctor said she was sitting up, eating, drinking and communicating with her family.
But Dr Michael Jacobs warned that she faced a “critical” few days while being treated with an experimental anti-viral drug which is “not proven to work” along with the blood from a survivor.
The hospital, where she has been treated in isolation since Tuesday, was unable to obtain ZMapp, the drug used to treat recovered British volunteer and nurse William Pooley, because “there is none in the world at the moment”.
Dr Jacobs said on New Year’s Day: “At the moment, we don’t know what the best treatment strategies are.
“That’s why we’re calling them experimental treatments.
“As we’ve explained to Pauline, we can’t be as confident as we would like.
“There’s obviously very good reason to believe it’s going to help her, otherwise we wouldn’t be using it at all, but we simply don’t have enough information to know that’s the case.”
Mrs Cafferkey, from Glasgow, was part of a 30-strong team of medical volunteers deployed to Africa by the UK Government last month and had been working with Save the Children at the Ebola Treatment Centre in Kerry Town, Sierra Leone.
She was initially placed in isolation at a Glasgow hospital early on Monday after feeling feverish, before being transferred south on an RAF C-130 Hercules plane.
The healthcare worker had flown from Sierra Leone via Morocco to Heathrow, where she was considered a high risk because of the nature of her work but showed no symptoms during screening and a temperature check.
But while waiting for a connecting flight to Glasgow she raised fears about her temperature and was tested a further six times in the space of 30 minutes.
Despite her concerns, she was given the all-clear and flew on to Scotland where, after taking a taxi home, she later developed a fever and raised the alarm.
The Government’s chief medical officer, Dame Sally Davies, admitted questions have been raised about the airport screening procedure for Ebola.
Scottish First Minister Nicola Sturgeon said: “Our thoughts continue to be with Pauline Cafferkey and her family during this extremely distressing time.
“I would like to thank all of the health professionals involved in treating Pauline, as they continue to show tremendous dedication and expertise.”