Downing Street has dismissed claims that 43,000 false Covid test results from a Wolverhampton laboratory are to blame for the sharp rise in the number of cases in the south-west of England, claiming that the region may be catching up with the rest of the country.

Experts have linked the high case rates in the west country to the problems at the Wolverhampton laboratory of a company called Immensa, which NHS test and trace suspended from processing PCR Covid tests earlier this month.

However, No 10 insisted on Monday that the false results issue was not the cause of the surge, suggesting that the lower Covid rates in the region in the past could be behind the current increase.

“In terms of the causes behind the increase in the south-west, we’ve seen there was this lab error; I don’t believe that accounts for the increases we have seen,” Boris Johnson’s official spokesperson said. “We know the south-west was an area that did not previously have as high rates as other parts of the country, which may be a factor as well.”

In response to the high rates of Covid, directors of public health in the south-west have advised schools to take measures such as cancelling assemblies, wearing masks in corridors and returning to bubbles.

The south-west currently has the highest case rate of any region, with 760 cases per 100,000, according to Public Health England. The south-east had the second-highest rate, with 526 cases per 100,000. The latest seven-day average figure for hospital admissions in the south-west was 91, up 14% on the previous week.

No 10 said it wanted to minimise disruption to learning, but backed public health directors to give such advice where necessary. “We would expect them to balance the public health risk with the longer-term harm of disruption to education,” the spokeperson said, adding that there were a “limited number of cases” where directors of public health were using their discretion to impose extra measures.

Asked later what the government was doing to hold Immensa to account, Johnson’s spokesperson said they were not aware of any specific plans to do so beyond the details of the contract. He added: “I’m not aware that specific issue is solely responsible for the uptick in case rates we’ve seen in the south-west region.”

Dr Kit Yates, a senior lecturer in the department of mathematical sciences at the University of Bath, said it was “inconceivable that telling 43,000 people they were negative when in fact they were positive, making them believe they could safely go in to schools and workplaces where they may have infected others, did not have an impact the prevalence of Covid in the south-west … In part we may be seeing the impact of people who were given the false negatives being asked to retest and finally appearing in the figures.

“However, the vast majority of people given the false negatives will no longer be testing positive, so this is unlikely to be a big driver of the case rates. These figures will be independent of the testing scandal, indicating that the fast rises we are seeing are genuine and not a result of retesting.”

He said two other factors might also help to explain the region’s high prevalence. “Firstly, the south-west has the highest proportion of the new AY4.2 subvariant, which seems to spread slightly faster than the original Delta variant.

“Secondly, it’s also the case that not all the false negatives were sent to the south-west; some were sent to Wales, some to the south-east and some to Yorkshire, so it’s possible that some of the rises in these regions are also due to the false negatives scandal.”

He also said it was “very odd to see No 10 trying to cover for this private company’s mistakes instead of demanding an urgent investigation and being upfront with the general public about what has happened”.

No 10 has so far resisted pressure from scientists and advisers to bring in “plan B” measures such as mandatory face masks on public transport and in shops, vaccine passports, and working from home.

Increasingly, however, some local authorities are using powers given to them by the government to issue their own advice on public health grounds. Dan Norris, the Labour mayor for the West of England, has sent out a poster urging people to wear face coverings to thousands of public-facing businesses.