Downing Street has not ruled out a “firebreak” lockdown as a last resort if the NHS were to be overwhelmed by Covid cases, but denied there were plans for one during October’s half-term school holiday.

With cases expected to rise further this autumn, some scientists have been warning that restrictions could be necessary in the coming months, such as limits on gatherings and a return to compulsory indoor masking.

A member of the Scientific Advisory Group for Emergencies (Sage) told the i paper that a “precautionary break” could be part of “contingency plans”, with another saying that “a firebreak lockdown is by no means out of the question”. They speculated this could take place during October half-term, with the break lengthened to two weeks rather than one.

Asked about the idea, Boris Johnson’s official spokesman said it was “not true that the government is planning a lockdown or firebreak around the October half-term”.

However, he made clear that further restrictions such as a two-week circuit breaker remain an option if the NHS were to be overwhelmed.

“We have retained contingency plans as part of responsible planning for a range of scenarios, but these kind of measures would only be reintroduced as a last resort to prevent unsustainable pressure on our NHS,” he said.

“I think we’ve been clear throughout that we will take action, and indeed we have done when necessary to protect our NHS. But under the previous occasions when that action has been required, we have been without the significant defences that our vaccination programme provides us – we’re now in a much different phase.”

Scientists said that the idea of a firebreaker had merit.

Ravi Gupta, professor of clinical microbiology at the University of Cambridge and a member of the Nervtag advisory body, but speaking in a personal capacity, suggested the plans may not go far enough.

“I guess my view is that this situation was entirely predictable and to some extent preventable and many scientists have been ignored,” he said.

“A firebreaker of a two-week half-term may help but is unlikely to be long enough to do the job, without additional measures including masking indoors and large gathering limitations,” he said. “Vaccinating 12- to 15-year-olds should help curb transmission but we have left it pretty late.”

Speaking in a personal capacity, Prof Julian Hiscox, chair in infection and global health at the University of Liverpool and a member of Nervtag said: “Every winter we have seen how influenza affects hospital admissions and we have this with a bigger scale with Covid-19.

“A firebreak, if necessary, is a good idea. The rising case rates and breakthrough in previously vaccinated people emphasises the need to administer a vaccine booster right now in preparation for winter and in parallel to the flu vaccine campaign. This will protect against serious disease and help keep hospital admissions down.”

Graham Medley, a member of Sage, and professor of infectious disease modelling at the London School of Hygiene & Tropical Medicine, said: “It’s entirely for government to decide whether it’s a good idea. It is one way of planning to reduce prevalence.”

Nadhim Zahawi, the vaccines minister, said he had not seen any plan for a firebreak lockdown. Asked on BBC Breakfast whether the government was looking at a possible lockdown in October, he said: “No, I haven’t seen any [plans] … I know where this question comes from. The i newspaper was reporting that they think there could be a possible lockdown. Look, vaccines have given us the ability to reduce infections, to save 100,000 lives.

“It is through the booster programme that I hope … we can transition the virus from pandemic to endemic status and deal with it, year in, year out. It is going to be with us for many years, but not [so we] have to close down our economy or take the severe measures we had to sadly take in December of last year.”

Cases were at more than 40,000 over the last 24 hours, and are expected to rise still further with the return of schools in England and Wales from their summer breaks. Scotland has already seen an increase in cases since its pupils returned.