It is the first time the UK variant, with a scientific name of B.1.1.7 and labelled by the World Health Organisation as Alpha, has been detected in Hong Kong.
The source of the girl’s infection remains unknown.
Dr Gilman Siu from the Polytechnic University’s Department of Health Technology and Informatics carried out the genome sequencing of the virus.
He said the case could be highly infectious.
“The variant with this mutation has been reported to be more transmissible. The patient is harbouring a very high viral load. That means it’s even more transmissible and very infectious,” he said.
It is the first untraceable coronavirus infection reported in Hong Kong after 42 days of zero cases.
Meanwhile, the government said no coronavirus cases were found at a Tin Shui Wai housing block where the girl lives.
The block, Shing Yu House at Tin Shing Court, was locked down on Friday night, during which 850 residents were tested.
Residents at the building will be tested again four more times in the next two weeks as a precaution.