Nearly 16,000 people who tested positive for the coronavirus in the UK weren’t included in the daily number of reported cases due to a technical glitch. This produced an artificially low estimate of the spread of the virus, delaying efforts to trace those that had been in contact with the infected people.

Public Health England (PHE) said 15,841 cases between 25 September and 2 October were left out of the UK daily case figures. They were then added in to reach Saturday’s figure of 12,872 new cases and Sunday’s 22,961 figure. All those who tested positive had been informed, but others in close contact were not.

The error was reportedly caused by the Excel spreadsheet used to compile the data. Michael Brodie, PHE’s interim chief executive, said that officials had identified an issue in the “data load” process which means the results were not uploaded to the national database. Some of the files apparently exceeded the maximum size that can be loaded onto PHE’s system.

A Downing Street spokesman said: “As NHS test and trace and PHE have said, a technical issue was identified on Friday in the process of transferring positive test results into the reporting dashboard. This was quickly resolved and all outstanding cases have been transferred into the contact tracing system.”

The news brought wide criticism on Prime Minister Boris Johnson, who has been on the defensive for its messy handling of the pandemic since March, when the government hesitated to impose a lockdown. More than 57,000 people have died from the virus in Britain, the highest number in Europe, and the country is now dealing with a second wave.

“This incident should never have happened,” the health secretary, Matt Hancock, said to Parliament, adding the government would do an investigation and upgrade its computer systems. Still, this didn’t appease the Labour Party, which described the episode as evidence of Johnson’s government serial incompetence. The party’s shadow secretary Jonathan Ashworth said the glitch was “shambolic.”

The UK reported 12,594 new cases yesterday, a number that did not include the backdated cases. A large part of these cases are in the northwest, in cities like Liverpool and Manchester, who have large university student populations. Adding the missing cases back into the reported data brought the rate of spread to within the government’s projections, officials said.

Almost immediately after England’s contact tracing program was unveiled in May, it has been plagued by problems regarding access to testing data, making it impossible in some cases for local officials to keep track of the virus. A network of testing sites processed tens of thousands of daily tests for weeks but the government didn’t share detailed results with local officials.