LONDON — Hospitals in England say a shortage of COVID-19 tests in the U.K. is jeopardizing medical staffing and preparations for a potential surge in coronavirus cases this winter.

Inadequate testing is leading to increased absences in the National Health Service as medical workers are forced to self-isolate while they and their family members wait for test results after possible exposures, according to NHS Providers, a group that represents hospitals. Last weekend hospital leaders in three different cities raised concerns about testing, said Chris Hopson, the group’s CEO.

“The problem is that NHS trusts are working in the dark — they don’t know why these shortages are occurring, how long they are likely to last, how geographically widespread they are likely to be and what priority will be given to healthcare workers and their families in accessing scarce tests,” Hopson said Tuesday.

The shortage comes amid a surge in COVID-19 cases across the U.K. that has pushed daily new infections to levels not seen since late May and has forced the Conservative government to impose new limits on public gatherings.

Widespread testing is seen as crucial to controlling the spread of coronavirus because it allows those who are infected to self-isolate while helping health officials to identify hot spots and trace those who are infected.

The problem is that the “second wave” of the virus is hitting Britain earlier than anticipated, said John Bell, a professor of medicine at the University of Oxford. Authorities have underestimated the speed at which more testing capacity is needed, Bell said, warning that the problem could get worse.

“I think what’s going wrong is the second wave,” Bell told the BBC. “A month ago, they had spare capacity in testing —significant spare capacity — but I think what has been underestimated was the speed at which the second wave would arrive.”

He also said new testing pressures are arising from children returning to school.

The government says it can process about 243,000 coronavirus tests a day, up from 220,000 at the end of August. Over the past week, many people have complained that they were being sent to testing centers far from their homes, sometimes hundreds of miles away.

Speaking to the House of Commons on Tuesday, Health Secretary Matt Hancock acknowledged that the U.K. testing system was facing “operational challenges.” He said the government is “working hard” to fix them — though it may take weeks to resolve all of the issues. The government is increasing testing capacity, and over the past week the average distance traveled to test sites dropped to 5.8 miles (9.3 kilometers) from 6.4 miles (10.2 miles), he said.

“We’ve seen a sharp rise in people coming forward for a test, including those who are not eligible,” Hancock said. “And throughout this pandemic we have prioritized testing according to need. … The top priority is and always has been acute clinical care.”

Hancock said he wouldn’t rule out further steps to make sure tests are used according to those priorities. But he faced a litany of complaints from lawmakers of all parties, furious that their constituents were unable to get the tests they need.

Moz Bulbeck Reynolds, who lives west of London, said she had been unable to schedule a test through the government website since Monday morning. Her 9-year-old daughter, Matilda, stayed home from school with cold symptoms on Thursday and Friday, and the school told her she couldn’t return without a negative test.

“I feel sorry for my daughter… rejected at the school gate,” Bulbeck Reynolds, 45, told Britain’s Press Association. “It made me feel like a failure as a parent.”

The government last week announced plans for an ambitious program, dubbed Operation Moonshot, that aims to administer millions of tests a day. But the estimated cost for the program nearly matches the whole NHS budget, Dr. Chaand Nagpaul says in a speech he plans to deliver Tuesday to the annual meeting of the doctor’s union.

“The government is now shooting for the moon, promising to deliver mass continuous testing with a test that doesn’t yet exist at a cost nearly as much as the total NHS budget,” he said. “Down here on Planet Earth, we need a fit-for-purpose test and trace system in the here and now.”

Alan McNally, director of the institute of microbiology and infection at the University of Birmingham, told the BBC there were “clearly underlying issues which nobody wants to tell us about,” together with an increased need for testing.

“I think there is a surge in demand (and) I think our stated capacity is very different from actually how many tests can be run in a given day,” he said. “It’s very worrying that we seem to be in a situation before really we’ve come into autumn and winter where we’ve maxed out the number of tests we can do.”