Contact tracers are scrambling to reach up to 50,000 people who should be self-isolating after ministers were accused of “putting lives at risk” with a spreadsheet blunder that led to coronavirus cases going unreported. MPs from across the political spectrum rounded on the health secretary, Matt Hancock, after it emerged that official figures missed 15,841 positive results due to a “catastrophic” data error. A million-row limit on Microsoft’s Excel spreadsheet software may have led to the error, which meant an estimated 50,000 people who were in close contact with someone with coronavirus had been circulating in the community for days when they should have been told to quarantine. About half of the 16,000 missed cases are thought to have been in north-west England, where the added cases saw Manchester’s infection rate double to more than 500 cases per 100,000 people as it became the worst-hit part of the country. Health officials were racing to track down contacts but only 51% of the 15,841 positive cases had been asked to hand over contact details by Monday morning, Hancock said. The government’s Scientific Advisory Group for Emergencies (Sage) advises that all close contacts should isolate within 48 hours if contact tracing is to be effective. Hancock told MPs: “This incident should never have happened. The team have acted swiftly to minimise its impact and now it is critical that we work together to put this right, and to make sure that it never happens again.” Prof Chris Whitty, the chief medical officer for England, had assessed the updated data, Hancock said, and concluded that no assessments of the spread of Covid had substantially changed. No local lockdowns would need to be reassessed, he added. Blaming a “failure in automated transfer for files”, Hancock said that no care homes, NHS sites or schools had been affected. But responding for Labour, the shadow health secretary, Jonathan Ashworth, said up to 48,000 contacts had been missed over a period of days. “Thousands of people blissfully unaware they have been exposed to Covid, potentially spreading this deadly virus at a time when hospital admissions are increasing and we’re in the second wave,” Ashworth said. “This isn’t just a shambles. It’s so much worse than this. And it gives me no comfort to say it, but it’s putting lives at risk, and he should apologise when he responds.” Hancock also faced some scepticism from his own benches over his argument that the data issue had been fixed and would not be repeated. Jeremy Hunt, the former health secretary who now chairs the Commons health committee, said the incident had shown an “underlying problem” of laboratories being overwhelmed by demand. Hunt told MPs the testing system should be fully revamped to shift testing for NHS and care staff to hospital and university laboratories, and warned Hancock against complacency. Bernard Jenkin, the Tory MP who chairs the influential liaison committee, also called for a rethink, suggesting that the military could assist with the process. “This is another incident which further undermines public confidence in the delivery of the government’s Covid response. It is another example of where logistics and planning have let us down,” he said. Experts said the error could increase Britain’s already rising rate of infections and put extra pressure on a testing system in England that was buckling under the strain of demand. “All those individuals with positive results that were not entered into the system have contacts who remained an infection risk to others over this period and so we can expect that they will have already contributed extra infections which we shall see over the coming week or so,” said Rowland Kao, a professor of veterinary epidemiology and data science at Edinburgh University. Local leaders made fresh calls for more control over the testing system. Joe Anderson, the mayor of Liverpool, accused ministers of a “pandemic of incompetence” that was damaging the region’s fragile economy. Nick Forbes, the leader of Newcastle city council, described the incident as “yet another catastrophic failure from an incompetent government that is moving recklessly from one avoidable disaster to another”. The mistake arose during the data processing Public Health England (PHE) undertakes to ensure that people who have tested positive for coronavirus are only counted once, even if they have had two or more tests, as some people have. Referring to the idea that the error might have been caused by the use of Microsoft Excel files to transfer data, Ashworth asked Hancock why a complex system costing £12bn had used an off-the-peg programme. The health secretary said a new contract had been signed in August, and work was under way to replace the system. But Ashworth said the failure epitomised problems with test and trace: “The prime minister told this house on 20 May we would have a world-beating system in place by June. It’s now October. The system is neither competent nor improving. Problems are getting worse. The government is failing on the basics. When will he finally fix this mess?” The failure meant 22,961 Covid cases were reported in Sunday’s figures, after 12,872 on Saturday. The error also meant the information was not passed on to data dashboards used for contact tracing, PHE said. A further 12,594 people were reported on Monday to have tested positive.