The change came hours after other top British health experts warned the public they must make more sacrifices to control the second wave of the COVID-19 pandemic.

In a briefing televised live to the nation, Chief Scientific Officer Patrick Vallance and Chief Medical Officer Chris Whitty said after a slow rise in new infections over the summer, the number of new COVID-19 cases is now doubling every seven days. They said new infections could increase tenfold to almost 50,000 a day next month if nothing is done now to stem the tide.

Whitty stressed that infection rates are rising among all age groups and said it’s not acceptable for individuals to ignore health guidelines or engage in risky activity. He said everyone must do their part to slow the spread of the disease because infections among the young and healthy will inevitably spread to friends, family and ultimately to the most vulnerable in society.

“This is not someone else’s problem,” he said. “This is all of our problem.’’

Britain already has Europe’s highest death toll in the pandemic, with over 41,800 deaths, according to a tally by Johns Hopkins University. Experts say all such figures understate the true impact of the pandemic, due to limited testing and other factors.

Almost 3,900 new infections were reported Sunday, a level not seen since early May. Britain’s highest daily number of new infections peaked at 6,199 cases on April 5.

While current death rates have remained relatively low so far, Whitty warned that deaths are likely to rise in the coming weeks. The U.K. reported a seven-day average of 21 deaths a day last week, compared with a peak of 942 deaths on April 10.

These numbers include only deaths that are directly related to COVID-19. The real toll could be much higher if emergency services are overwhelmed by coronavirus cases and the National Health Service has to divert resources from treating other diseases, Whitty said.

But Whitty said new restrictions have to be balanced against the impact on the economy and society, because increased deprivation and mental illness will also lead to deaths.

“Ministers making decisions — and all of society — have to walk this very difficult balance. If we do too little, this virus will go out of control and you will get significant numbers of increased direct and indirect deaths,’’ he said “But if we go too far the other way, then we can cause damage to the economy, which can feed through to unemployment, to poverty, to deprivation.”

Vallance said efforts to find a vaccine are continuing and while it is still not clear they will work, the research is pointing in the right direction. He said some vaccine could be available by the end of the year “in small amounts for certain groups.”

The British government last week stopped short of imposing a total lockdown in northeastern England, where the infection rate first began to rise. Instead, bars and restaurants were ordered to close at 10 p.m. and people were prohibited from socializing with other households.

Lockdown fears hit stocks on Monday as shares in companies in aviation, hospitality and travel took a beating. Shares in IAG, the parent company of British Airways, fell 11.3%, while pub company Mitchells & Butlers fell 11% and Intercontinental Hotels Group fell 4%.

Dr Michael Head, a global health expert at the University of Southampton, noted that Monday’s warning highlighted the worst possible situation and did not take into account local lockdowns that could stem the spread of the virus.

“It is important to note that the chief medical (officer) did say the potential for 50,000 cases a day is a ‘worst-case scenario’,” he said, but added “it’s a timely reminder that the pandemic is accelerating, both globally and also here in the U.K.”

The rise in U.K. infection rates comes as lawmakers across the political spectrum have criticized the Conservative government’s testing program as inadequate. While ministers tout the record numbers of tests being performed, there are widespread reports of people having to travel hundreds of miles for tests or tests being voided because it’s taking labs too long to process them.

An app meant to bolster virus contact tracing efforts is to be released this week after months of delay.

In hopes of persuading people to stay home, the government announced it would pay low-income people 500 pounds ($639) if they are asked to self-isolate for 14 days. It also warned those breaking quarantines of possible fines that could run into thousands of pounds (dollars).