A newly detected coronavirus variant is on the rise in England, with the virus believed to be an offshoot of Delta.

According to a briefing from the UK Health Security Agency, released on Friday, “a Delta sublineage newly designated as AY.4.2 is noted to be expanding in England”, with the body adding that the variant is being monitored and assessed.

The report states that in the week beginning 27 September – the last week for which complete sequencing data was available – AY.4.2 accounted for about 6% of sequenced coronavirus cases and is “on an increasing trajectory”.

AY.4.2 contains two mutations in its spike protein, called A222V and Y145H. The spike protein sits on the outside of the coronavirus and helps the virus to enter cells.

However, Ravi Gupta, a professor of clinical microbiology at the University of Cambridge, told the Guardian these mutations were not of particular concern. “A222V has been seen in other lineages of Delta,” he said. “It doesn’t have a really large effect on the virus.”

Gupta added similar mutations to Y145H had been seen in the Alpha variant and other variants. While these appear to have an effect on the binding between antibodies and the virus, Gupta said the effect was, at most, modest.

“But in my mind, these are not really significant mutations,” he said. “We need to look further at what the non-spike mutations are.”

In the past, new Covid variants have fuelled large surges in the number of cases – the Alpha variant played a key role in rising cases last winter, while the Delta variant took off and led to sharp increases in the spring. Daily Covid cases in England are rising once again: 39,624 new Covid cases were reported on Saturday, the highest figure since late July.

According to the Financial Times, Dr Jeffrey Barrett, the director of the Covid-19 Genomics Initiative at the Wellcome Sanger Institute in Cambridge, and Prof Francois Balloux, the director of the University College London Genetics Institute, have suggested AY.4.2 could be 10-15% more transmissible than the original Delta variant.

However, they urged caution. “Britain is the only country in which it has taken off in this way and I still would not rule out its growth being a chance demographic event,” Balloux told the FT.

Balloux told the Guardian the new variant was unlikely to be behind the recent rise in cases.

“Its potential higher transmissibility could at this stage only explain a tiny fraction of additional cases,” he said. “It’s around 10% frequency [now], and assuming it may be 10% more transmissible, that would only explain an additional 1% extra infections [every five or so days].”

Gupta said the focus on a new variant was missing the point, noting a number of other important factors including the slow rollout of jabs to older children. According to official figures, vaccination rates among 12- to 15-year-olds were about three times lower in England than Scotland.

“We shouldn’t be blaming the virus for what is going on in the UK,” said Gupta. “It is because we have fundamentally failed to control transmission, and that is because kids are vulnerable, they have not been vaccinated, they are back at school, they are spreading virus among themselves and they are feeding it into their families.”