Covid warning: Experts baffled as 'unusual' Delta variant rips through UK: 'Not good news'

He said: “This lineage is much more common outside of the UK, so there could be a variety of introductions from abroad that have coincidentally grown a bit.

“It has been growing in Denmark, France, Belgium and Germany, which could be a biological advantage, or just the fact that those countries are being hit with Delta case growth.”

The AY.43 subvariant carries a unique mutation known as N:Q9l, which Dr Jeffrey dubbed “unusual” but there is no evidence yet to suggest it makes the virus more transmissible than Delta.

A more worrying subvariant is the A.Y.4.2 or “Delta Plus” virus which, according to researchers at Northumbria University, can be traced to the first Delta lineages discovered in April this year.

Last month, Northumbria researchers Matthew Bashton and Darren Smith said scientists have already identified 75 AY lineages of Delta, each with “additional defining mutations in their genome”.