Health officials in Britain said they were investigating a rise this week in the number of Covid-19 tests results that have a trait that is associated with the newly identified Omicron coronavirus variant of concern.

One of Omicron's mutations produces S-gene target failure (SGTF) in about half the PCR tests in use in Britain, allowing it to be distinguished from Delta and providing a clue over the spread of Omicron before full genomic sequencing.

Although a useful early indicator of the spread of Omicron, SGTF is associated with other variants too, such as Alpha, Reuters reports.

“The proportion of test results displaying SGTF has been very low in recent months but an increase has been observed in the past week,” the UK Health Security Agency said.