Rising COVID-19 cases heading into winter "suggest this peak could be as serious as the last", the UK's former chief scientific adviser has warned.
Sir David King was speaking to Sky News a day after 49,156 cases were reported - the highest since mid-July.
Cases have been regularly over 40,000 in the last week - with the high numbers mainly being attributed to infections among schoolchildren.
"The disease is rising to another peak, and this peak could be as serious as the last," Sir David told Sky's Kay Burley.
It comes as a former World Health Organization director claimed the UK had the highest COVID-19 case rates in the world and a death rate far higher than China.
Professor Anthony Costello accused the government and advisers of being "silent" on the issue.
Downing Street said on Monday that the winter months look "challenging" and that it was keeping a "close watch" on cases.
However, it said new infections were roughly in line with predictions and that the vaccine programme had "substantially" broken the link between cases, hospitalisations, and deaths.
Watch: COVID-19 - Warning that UK's next peak 'could be as serious as the last' as cases hit three-month high
The latest seven-day average for deaths is 124.1, while the latest count for people in hospital for COVID is 7,097.
Both are much lower than the winter peak in January, and cases remain below the initial prediction of 100,000 per day made by the health secretary ahead of England's so-called Freedom Day.
But there are fears these numbers could rise significantly in the coming months.
Sir David also criticised the booster jab rollout as going "extremely slowly".
"I do know many older people who haven't had their booster - I can't understand this. If the vaccines are available, what's happening to the rollout?" he said.
A third COVID-19 jab is to be offered to groups including over-50s, vulnerable groups, and people who work in care homes and frontline health care.
The NHS will contact those who are eligible, so long as it's been at least six months since their second jab.
The UK's former science chief also expressed frustration at lower levels of mask wearing now most COVID rules have been scrapped.
"How many people are still wearing masks? And for goodness’ sake, why not?" said Sir David.
"I just don't understand why when I get into a train or an Underground, I don't see everybody wearing masks. Why aren't we still requiring people to travel with masks, to go into buildings with masks?
"It just seems a very simple thing."
Watch: COVID-19: Minister rules out another lockdown as PM is urged to enforce 'Plan B' to avert winter crisis
He also cautioned over the thousands of people coming to the UK for the COP26 climate summit in Glasgow, calling it a "massive potential rollout of the disease".
"It's a viral factory to put all those people together from 197 nations of the world," he said.
"I really worry about it - not because they will bring disease into use, but because we are the country with a very high level of virus, and we will be passing on to them."
The government last month set out a Plan A and B for dealing with COVID over winter, as it hopes to avoid more lockdowns.
Plan A includes expanding the vaccine rollout, such as to school-age children, plus more cash for the NHS COVID response, and a focus on antiviral drugs.
It also stresses that NHS Test and Trace and self-isolation will continue to be "critical" in keeping the disease under control.
Plan B would kick in should the NHS be threatened by "unsustainable pressure". England's chief medical officer, Professor Chris Whitty, has already warned the health service faces an "exceptionally difficult" winter.
Measures could include the possibility of vaccine passports in some settings in England - such as nightclubs and large sports events, while mask rules and working from home may also return.