Britain's health minister on Monday blamed out-of-date software for the fact that almost 16,000 positive coronavirus tests went missing from official figures, and admitted it had affected contact tracing efforts.
Matt Hancock said the "legacy computer system" at the Public Health England (PHE) agency that processed lab results caused 15,841 cases between September 25 and October 2 to be absent from reported daily case numbers.
"Our assessment of the disease and its impact has not substantially changed as a result of this data," he insisted, adding that everyone who tested positive received their result as normal.
But he conceded the glitch disrupted the government's test and trace programme, which aims to find close contacts of coronavirus sufferers to ensure they self-isolate.
A catch-up effort after the fault was reported on Friday night had so far contacted 51 percent of those cases, he told the House of Commons.
"This incident should never have happened but the team has acted swiftly to minimise its impact," he said.
However Jon Ashworth, from the main opposition Labour party, said it was just the latest sign that the tracing system Prime Minister Boris Johnson promised would be "world-beating" was not up to the task.
"This isn't just a shambles… it's putting lives at risk," he said.
Sunday's official coronavirus numbers showed 22,961 new positive cases, a rise of more than 10,000 on Saturday's figures, bringing the total to more than 500,000.
More than 42,000 people confirmed to have Covid-19 have died in Britain, the worst toll in Europe.