Boris Johnson has been warned to implement Covid "Plan B plus" for the winter or risk an NHS crisis.
NHS Confederation boss Matthew Taylor, which represents the healthcare systems in England, Wales and Northern Ireland, said the new curbs were needed or the UK would "risk stumbling into a winter crisis".
It comes after deaths from Covid peaked at 223 - the highest seen for months - while the seven-day average for Covid-19 cases stands at 44,145 a day.
The government has warned that working from home and vaccine passports could be introduced if the NHS is at risk of being overwhelmed.
But Business Secretary Kwasi Kwarteng ruled out imposing another lockdown - and even told Brits to go ahead with planning Christmas parties.
Mr Taylor said action needs to brought in now or it will derail efforts to tackle the backlog of five million patients waiting for treatment.
He said: "We are right on the edge – and it is the middle of October. It would require an incredible amount of luck for us not to find ourselves in the midst of a profound crisis over the next three months.
"The government ought to not just announce that we’re moving to Plan B, but it should be Plan B plus.
"We should do what’s in Plan B in terms of masks… working from home, but also we should try to achieve the kind of national mobilisation that we achieved in the first and second waves, where the public went out of their way to support and help the health service."
As he urged people to avoid risky behaviour, Mr Taylor said inaction would lead to a "really profound crisis" in the health and social care through the winter.
No10 said it was keeping a "very close eye" on rising case rates, but added the Prime Minister has "absolutely no plan to introduce Plan B" of its winter plan.
Mr Kwarteng insisted that another lockdown would not be imposed.
"No, I would rule that out," he told Sky News.
"Throughout this process, there've been people saying the lockdown was unnecessary, there have been other people saying we should continue the lockdown. We've really plotted a path between those two extremes."
But he said it was a "good thing" for people to wear masks in public places.
"I think people should do what they feel is the right thing to do," he said. "They've got to, I think, be respectful towards other people, they've got to keep themselves safe and the public as well."
Mr Kwarteng also struck an optimistic note when he told Brits it was "absolutely" worth it to pay a deposit on a Christmas party.
Asked on Times Radio whether it was wise to pay money towards a party, he said: "Absolutely. I think I already have, we've got some arrangements in place and I look forward to having a Christmas party as usual."
Meanwhile, one of the architects behind the first lockdowns has warned restrictions may need to be brought back in.
Professor Neil Ferguson, a member of the Scientific Advisory Group for Emergencies (Sage) from Imperial College London, said Covid hospitalisations are doubling every five weeks as immunity wanes.
Just 3.7 million of the 8.5 million at-risk people who had their second jab at least six months ago have had their third dose, and the gap is growing.
Professor Ferguson said: “I don’t think we’re looking at another lockdown... the worst case here are demands on the NHS... it’s very unlikely we’ll see anything like the levels of deaths we saw last year, for instance.
“Coming into the winter, there may be a Plan B which needs to be implemented, which involves some rolling back of measures, but I doubt that we’ll ever get close to lockdown we were in January of this year.”
He added: “I don’t think it’s a reason to panic right now but I would certainly like to see vaccination booster doses accelerated, vaccination for teenagers accelerated.”
