The United Kingdom could see about 50,000 new COVID-19 cases a day by mid-October if the current rate of infection is not curbed, the government's chief scientific adviser warned Monday.

"At the moment we think the epidemic is doubling roughly every seven days," Sir Patrick Vallance said in a televised address from London. "If, and that’s quite a big if, but if that continues unabated and this grows doubling every seven days... if that continued, you would end up with something like 50,000 cases in the middle of October per day."

That rate of infection would be expected to lead to 200-plus deaths per day by mid-November, according to Vallance, who noted that there are already measures in place to prevent the country from hitting those grim milestones.

"That requires speed, it requires action, he said, "and it requires enough in order to be able to bring that down."

Vallance said the increase in COVID-19 infections has been among "every age group" and that the number of people in the country showing antibodies for the disease remains low, meaning the "vast majority of the population remain susceptible."

"As the disease spreads, as it spreads across age groups, we expect to see increasing hospitalizations," he added. "And unfortunately, those increasing hospitalizations will lead to increasing deaths."

California's death toll from COVID-19 has surpassed 15,000, according to a real-time count kept by Johns Hopkins University.

The state's tally of coronavirus-related fatalities, which currently stands at 15,016, is the fourth-highest in the country, after New York, New Jersey and Texas.

California has reported the most COVID-19 infections of any U.S. state since the start of the pandemic, with more than 786,000 confirmed cases.

However, the state's infection rate has fallen to 3% in the past week, the lowest level since the early days of the pandemic, according to The Associated Press.

People in England who violate an order to self-quarantine will face fines of up to 10,000 British pounds, amid an alarming rise in COVID-19 infections.

British Prime Minister Boris Johnson said the new penalties mean people "are legally obliged" to self-isolate if they test positive for COVID-19 or are traced as a close contact to someone who did. The fines, which take effect next week, will start at 1,000 British pounds (approximately $1,300) but could increase to up to 10,000 pounds (about $13,000) for repeat offenders.

The higher fines could be applied to "the most egregious breaches," including those who prevent others from self-isolating, such as business owners who threaten employees with losing their jobs if they don't come into work.

Low-income workers who face a loss of earnings as a result of having to self-quarantine will be eligible for a one-time support payment of 500 British pounds (approximately $650).

The new fines will come into force in England on Sept. 28. Officials are in talks with the devolved governments of Northern Ireland, Scotland and Wales about expanding them U.K.-wide.

"The best way we can fight this virus is by everyone following the rules and self-isolating if they're at risk of passing on coronavirus," Johnson said while announcing the new rules over the weekend. "People who choose to ignore the rules will face significant fines. We need to do all we can to control the spread of this virus, to prevent the most vulnerable people from becoming infected, and to protect the NHS and save lives."

An official estimate shows that new COVID-19 infections and hospital admissions are doubling every seven to eight days in the United Kingdom. There were 3,899 new infections and 18 fatalities reported Sunday, bringing the country's tally to 394,257 cases and 41,777 deaths, according to the latest figures from the U.K. government

ABC News' Christine Theodorou contributed to this report.

An additional 230 coronavirus-related fatalities were recorded in the United States on Sunday, as the country's death toll inches closer to the 200,000 mark, according to a real-time count kept by Johns Hopkins University.

Sunday's tally of COVID-19 deaths is well under the country's record set on April 17, when there were 2,666 new fatalities in a 24-hour reporting period.

There were also 38,978 new cases of COVID-19 confirmed across the nation on Sunday, down from a peak of 77,255 new cases reported on July 16.

A total of 6,805,630 people in the United States have been diagnosed with COVID-19 since the pandemic began, and at least 199,512 of them have died, according to Johns Hopkins. The cases include people from all 50 U.S. states, Washington, D.C. and other U.S. territories as well as repatriated citizens.

By May 20, all U.S. states had begun lifting stay-at-home orders and other restrictions put in place to curb the spread of the novel coronavirus. The day-to-day increase in the country's cases then hovered around 20,000 for a couple of weeks before shooting back up and crossing 70,000 for the first time in mid-July. The daily tally of new cases has gradually come down since then.

Week-over-week comparisons show that the number of new cases and the number of new deaths recorded in the United States are both decreasing, according to an internal memo from the Federal Emergency Management Agency obtained by ABC News last week.