Coronavirus testing will be rolled out to people working in public services such as police, fire and prison staff, Health Secretary Matt Hancock has said.
Capacity was rising “sharply” but not as many NHS staff had come forward for tests as had been expected, he said.
No 10 said 18,665 tests were carried out in the 24 hours to 09:00 BST on Thursday but there had been capacity for at least 38,000.
Meanwhile, a health expert said the crisis could lead to 40,000 UK deaths.
Prof Anthony Costello, the director of University College London’s Institute for Global Health, told an online meeting of the health select committee that the UK was “too slow” to react on a number of fronts to the crisis which may have led to it having “probably the highest death rates in Europe”.
Sir Patrick Vallance, the government’s chief scientific adviser, has said it would be a “good outcome” if total UK deaths could be kept below 20,000.
Warning that the UK would face “further waves” of the virus, Prof Costello said a system needed to be put in place that “cannot just do a certain number of tests in the laboratory” but one that reached out at “district and community level”.
Mr Hancock said the government had prioritised testing for hospital patients and NHS workers before expanding it to residents and staff in social care.
He added some 50,000 NHS workers had so far been tested.