The UK Health Security Agency has designated the Omicron variant sub-lineage, known as BA.2, as a variant under investigation – with current case rates very low.
Just 53 sequences of the sub-lineage had been identified in the UK by 10 January, the HSA said.
The health body said further analysis will be undertaken.
BA.2 does not contain the spike deletion and therefore is S-gene positive (SGTP) – making it harder to identify as Omicron than the original, dominant strain, BA.1.
S-gene target failure (SGTF) is a useful indicator of Omicron, because as a rule Delta cases have the S-gene and Omicron cases do not.
The HSA said last week that an increasing number of BA.2 sequences had been found in recent weeks, many from Denmark.
It said 2,093 sequences had been found from 22 countries including the UK.