Public Health England is investigating cases of coronavirus with 'worrying' new genetic changes that have been found in some regions of the UK.
Tests show they have a mutation, called E484K, that is already seen in the South Africa variant.
Although this change may reduce vaccine effectiveness, the current ones in use should still work, say experts.
There have been 11 cases in Bristol and a cluster of 32 cases in Liverpool.
Scientists working with Public Health England found a small number of cases of the UK 'Kent' variant with the E484K mutation - it was seen in 11 out of 214,159 samples that they tested, and predominantly from the South West of England.
It is likely there may be more cases that haven't yet been found. The Liverpool area has seen 32 cases of original coronavirus that have the E484K mutation too.
It's not unexpected that variants are appearing or that they will continue to change - all viruses mutate as they make new copies of themselves to spread and thrive.
Dr Julian Tang, a virus expert at the University of Leicester, described the finding as "a worrying development, though not entirely unexpected".
He said it was important people follow the lockdown rules and get cases of coronavirus down to prevent opportunities for the virus to mutate further.
"Otherwise not only can the virus continue to spread, it can also evolve."
He said that allowing spread could allow a "melting pot" for different emerging variants.
Cred: Michelle Roberts
Health Editor, BBC News Online