LONDON (AP) — The British government has launched an investigation into how nearly 16,000 new coronavirus infections went unreported as a result of a technical glitch, a failing that could have given fresh impetus to an outbreak that critics say could easily get out of control.

Health Secretary Matt Hancock told lawmakers Monday that the problem related to the “automated transfer of files.” The problem is widely thought to be connected to the file size limitations on Excel spreadsheets used in the test-and-trace program.

“This is a serious issue which is being investigated fully,” Hancock said. “Now it is critical we work together to put it right and make sure it doesn’t happen again.”

Hancock’s appearance at the House of Commons came after the weekend disclosure that 15,841 virus cases weren’t tabulated from September 25 to October 2. Though those testing positive had been told of their status, their contacts — potentially around 50,000 people — weren’t traced, a failing that could have allowed the virus to flourish.

He said 51 per cent of those testing positive were contacted again and that their contacts were reached immediately after that.

Despite the addition of so many new cases, he said the government’s chief medical officer “has not substantially changed” his opinion about the epidemic’s path in the UK, which like other countries in Europe is witnessing a second spike of the virus.

Jonathan Ashworth, Hancock’s counterpart in the main opposition Labour Party, said the failing showed how “shambolic” the Conservative government’s plan to fight the pandemic was and that the latest problem afflicting the country’s testing regime was “putting lives at risk.”

Lawmakers from all parties have criticized Prime Minister Boris Johnson’s government for a shortage of testing capacity that’s meant some people have been asked to travel hundreds of miles for a test, and delays in notifying people of their test results.

The latest problem appears to have been caused by an Excel file maxing out during an automated process. Though the software is a staple in consumer settings, experts say it has a number of limitations for use in much grander projects.

“If this was Excel as is being reported, the limitations of Excel, which is generally a very decent piece of software, in terms of Big Data are well known,” said Jon Crowcroft, a professor of communications at the University of Cambridge.

For the test-and-trace program to work well, contacts should be notified as soon as possible, within hours preferably. So authorities’ failure to inform people potentially exposed to the virus could lead to many more positive cases and the subsequent need for the government to impose further unwanted restrictions on everyday life.

Ashworth slammed the government for its latest failing “at one of the most crucial points in the pandemic,” adding that the contacts of those unreported new cases may have been “blissfully unaware they’ve been exposed to COVID, potentially spreading this deadly virus at a time when hospital admissions are increasing and we are in the second wave.”

“This isn’t just a shambles — it’s so much worse than this — and it gives me no comfort to say this, but it’s putting lives at risk,” Ashworth added.

The unreported cases were added to the government’s daily new infections total over the weekend, boosting Saturday’s number to 12,872 cases and Sunday’s to 22,961. That compared to an average of 7,000 new cases a day the four days before. The number of new cases reported Monday fell to 12,594, but given the adjustments related to the missing cases, it was impossible to figure out a trend.

Paul Hunter, a professor in medicine at the University of East Anglia, called the glitch “very disappointing.”

“For the test, track and trace system to have a real impact on reducing transmission of COVID-19, it is essential that test results are communicated rapidly,” he said.

Like other countries in Europe, the UK has seen rising coronavirus infections over the past few weeks, which has prompted the government to announce a series of restrictions, both nationally and locally, to keep a lid on infections. The new rules limit the number of people allowed to gather together and put a curfew on pubs.

The UK has Europe’s highest virus-related death toll at around 42,400. The government’s chief scientific advisers warned two weeks ago that if more restrictions were not taken, the country could end up with 50,000 new cases a day by mid-October, leading to hundreds of daily deaths a month later.

The confusion over the daily testing numbers only adds to the uncertainty over whether the restrictions are working in suppressing the virus.