Pauline Cafferkey, 39, of Glasgow is the first person to have been diagnosed with the virus on UK soil.
The Royal Free Hospital said her condition "has gradually deteriorated over the past two days and is now critical."
The hospital said Wednesday that Cafferkey had decided to have blood plasma treatment -- using plasma from Ebola survivors -- and to take an experimental antiviral drug.
British Prime Minister David Cameron tweeted that his thoughts and prayers are with Cafferkey.
The Royal Free Hospital is equipped with a high-level isolation unit where access is restricted to specially trained medical staff. A specially designed tent with controlled ventilation is over the patient's bed.
Another British volunteer nurse, William Pooley, was treated in the unit after his return home from Sierra Leone in August after being diagnosed with Ebola
British media outlets have said Cafferkey is a public health nurse who was part of a 30-strong team of medical volunteers deployed to Sierra Leone by the UK government last month in a joint endeavor with the charity Save the Children.
Cafferkey traveled back from Sierra Leone via Casablanca, Morocco, and London Heathrow Airport before arriving at Glasgow Airport on a British Airways flight late on December 28, the health agency NHS Scotland said.
After feeling unwell, she sought medical attention and was transferred to London on a military aircraft fitted with an isolation pod.
Amid concerns about the possible spread of the disease, authorities have been working to contact all those who may have come into contact with Cafferkey as she traveled back to Scotland.
Public Health England, a government agency, said it had contacted all 101 UK-based passengers and crew who flew from Casablanca to Heathrow, while its Scottish counterpart had reached all 71 passengers and crew members who traveled from Heathrow to Glasgow.
"Passengers given advice & reassurance. Additional 31 international passengers being contacted by international public health authorities," Public Health England said on Twitter.
The government has said the risk of infection to other passengers is "considered extremely low."
The number of deaths in the three West African countries where the outbreak is centered has climbed to 7,989 as of December 31, the World Health Organization said Friday.
There have been more than 20,000 confirmed, probable and suspected cases in Sierra Leone, Guinea and Liberia, the WHO said.