Prof Andrea Sella of University College London said the discovery was not surprising. “If someone deliberately mucks up the protocol then of course you’ll get a duff result. But I would add that it’s not a ‘false positive’ in the true sense. Because false positives are ones that take place in spite of adherence to the protocol.”

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Jon Deeks, a professor of biostatistics at the University of Birmingham, criticised the practice. “False positives affect not just that child but their family and their bubble at school, so [it is a] pretty selfish thing to do. There are less harmful ways to fake a day off school,” he said.