Further variants of coronavirus have been identified in the UK.
Virus threats advisory group NERVTAG has described a new strain of Covid found in Bristol to be "of concern".
The strain is understood to be a mutation of the Kent variant, which was first detected in September 2020.
Additional community testing has begun in Bristol and South Gloucestershire to help limit the spread of infection.
How many cases are there of the new Covid variant?
There are 21 known cases of the Bristol Covid variant, 15 of which were found in the South West and six discovered in other parts of England.
Meanwhile, a similar strain of the Kent mutation in Liverpool has also been identified and has been classed as a "variant under investigation".
Health officials have found 76 cases of the two variants first identified in Bristol and Liverpool as of Tuesday 9 February.
Will Covid vaccines still work against the Bristol variant?
Concern around emerging variants comes at a time when the rollout of approved vaccines is on course to reach 15 million people by 15 February.
The Bristol variant possesses the E484K mutation, also found in the South African variant, which helps the virus avoid the body's immune defences.
NERVTAG believes vaccinated people should still be protected against severe illness caused by the variants.
The team that developed the Oxford / AstraZeneca jab is said to be working on updating its vaccine to make it more effective against mutations.
Dr Susan Hopkins, from PHE, said: "I think it’s important to note that the E484K mutation in particular has been emerging and disappearing over time in multiple lineages and multiple variants since April last year.
"But clearly with it on top of what we know about the transmissibility of B117, the dominant variant in the UK, we’re monitoring it with particular concern."
What is NERVTAG?
NERVTAG is an acronym of New and Emerging Respiratory Virus Threats Advisory Group, which is an expert committee.
It is an arm of the Department of Health and Social Care (DHSC) which advises the UK government's chief medical officer, professor Chris Witty.
The advisory group has identified four mutations of the deadly coronavirus disease as "variants of concern"
These include variants from Kent, a new Kent variant found in Bristol, South Africa, and one from Brazil which is yet to have been found in the UK.
What Covid symptoms should people look out for?
Like with all variants, the main symptoms of Covid are a high temperature, a new and continuous cough or a loss or change to sense of smell or taste.
However, due to the increased transmissibility of the Bristol variant, everyone in dedicated postcodes (listed below) is being asked to get tested.
This is to limit the spread of the virus by finding asymptomatic cases as well and prompting people to self-isolate.
A number of additional mobile test sites have been set up for people not showing symptoms, as well as others for people with symptoms.
Find out where on the Bristol City Council website.
Which Bristol postcodes are being tested?
Residents living in the following postcodes can get tested:
BS8 3 (only within the Bristol City Council boundary, check your postcode on GOV.UK)
BS14 0 (only within the Bristol City Council boundary, check your postcode on GOV.UK)