One of them, who was described by Sturgeon as a "low probability" case in Scotland, has tested negative for the disease. The health service said the second possible case was in Cornwall, England, and details were not available on the third.

The World Health Organization said on Monday that the number of people infected by Ebola in the three West African countries worst affected by the outbreak - Sierra Leone, Liberia and Guinea - had passed 20,000, with more than 7,842 deaths so far.

Cafferkey, a National Health Service worker who had been working in West Africa with the charity Save the Children, flew from Sierra Leone to Glasgow late on Sunday on a British Airways flight via Casablanca in Morocco and London's Heathrow.

Health officials said she was screened for a high temperature with other returning health workers at Heathrow but showed no signs of fever. She requested further screening at the airport but six subsequent checks were within normal levels.

She was diagnosed with the virus on Monday after developing symptoms overnight and was initially treated at a Scottish hospital.