LONDON (Reuters) - A Scottish nurse who contracted Ebola in Sierra Leone last year, recovered and then suffered a relapse, has improved slightly to a serious but stable condition, hospital officials said on Monday.

Pauline Cafferkey, 39, was transferred from the Queen Elizabeth University Hospital in Glasgow to an isolation unit at the Royal Free Hospital in London on Oct. 9, and was last week described by her doctors as critically ill.

On Monday, however, the London hospital issued an update saying: "Pauline Cafferkey's condition has improved to serious but stable." It gave no further details.

Cafferkey, from South Lanarkshire, Scotland, spent several weeks in an isolation unit at the Royal Free at the beginning of the year after contracting Ebola virus in December 2014.

She was the first person to have been diagnosed with Ebola on British soil.

She was discharged in January after seemingly making a full recovery but then suffered a relapse earlier this month, with doctors saying last week she was being treated for Ebola in the hospital's high level isolation unit.

Infectious disease specialists say her case - the first time someone has been known to have recovered from Ebola hemorrhagic fever and then suffer an apparently life-threatening relapse - is taking them into uncharted waters but will hopefully reveal more about the deadly virus.

(Reporting by Kate Kelland, editing by Angus MacSwan)