The largest laboratory company in the US has warned it will be impossible to increase coronavirus testing capacity to cope with a surge in demand during the autumn flu season, in a sign that crippling delays will continue to hamper the American response to the pandemic.

The stark warning comes as the country struggles to tackle a resurgence of the disease, especially in sunbelt states like California, Texas and Florida, even as some countries in Europe and Asia have had some success in taming the virus.

Long delays in processing test results — which are taking more than a week to return — are exacerbating the situation and the time lag is expected to worsen in the autumn, when millions of Americans catch common colds and the flu.