The relatively slow rise in cases of the South African and Bristol variant is “reassuring” but controlling them will become much more challenging as lockdown is relaxed, a leading medic has said.

Dr Susan Hopkins, from Public Health England, said the transmission of both variants remained fairly “static” as the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), a group of scientists which advises government, added the variant detected in Bristol to its “variant of concern” list.

Meanwhile a variant identified in Liverpool has been classed as a “variant under investigation”, the group added.

Health officials said they had so far found 76 cases of the Bristol and Liverpool variants in the UK.

Both the variants found in Bristol and Liverpool contain the E484K mutation, a genetic change also found in both the South African and Brazilian variants, which experts suggest may be better at evading the human immune response.

The mutation detected in Bristol is to the B117 UK variant, which is the more transmissible strain that was first found in Kent and is now the dominant strain across the country.

Dr Hopkins told a briefing for journalists it was reassuring that cases of the South African variant, or the one detected in Bristol were not quickly rising.

She said: “I think it is reassuring.

“And we aren’t seeing week on week increases in either of them, they’re very much staying static.

“Of course the data is always lagging a little bit behind, but we’ve been watching these now for four weeks so I think that is reassuring.

“I think where we’re getting on top of things now, while we’re trying to control them, is because R is below one and we have a chance of doing something.

“As we start to release in March, April, May, this will become much more challenging.

“So everything we do now prepares us for then.”

Dr Hopkins added 170 cases of the South African variant had been identified so far in the UK, including 18 that are not linked to travel.

“To date we have identified 170 cases and 18 of these are unlinked to travel, and that means that they neither travelled abroad themselves or had direct contact with an individual that has travelled,” she said.

Department of Health and Social Care officials said they have “a high degree of confidence that the vaccines will work against variants”.

There are now four “variants of concern” of the virus that causes Covid-19 identified by government advisers, three of these have been found in the UK, and the fourth is the Brazil variant identified in people who had travelled to Japan.

This has not been detected in the UK.

Officials are also tracking two “variants under investigation”, these include the Liverpool variant and a second Brazil variant.

Analysis is ongoing to establish the impact of these mutations on the virus.

Experts do not think that the Kent variant with the E484K mutation is going to be more transmissible or pose more severe illness.

But studies suggest the South African variant may be able to escape the body’s immune responses, potentially making vaccines less effective against it.

Nick Loman, professor of microbial genomics and bioinformatics, University of Birmingham said he did not think the South African Variant was likely to become more dominant than the UK variant.

He explained: “We haven’t seen anything quite like the UK variant in terms of its growth rate, in terms of its transmissibility, so I’m going to make the assumption that the South African strain will not be able to out compete that variant.”

Saying that monitoring would continue he added: “If I was going to put a bet on, I would bet that it doesn’t out compete the B117 variant.”

It comes as officials said extra coronavirus testing will be carried out in Lambeth, south London, following the detection of the South African variant.

The surge testing and genomic sequencing will be carried out in the SE27 0, SE27 9 and SW16 2 postcodes.