The problems in testing appear to have been caused by a surge in demand, leading to laboratories buckling under the strain of trying to process tests.
This has led to testing slots being curtailed as laboratory staff work through the backlog.
Health Secretary Matt Hancock told MPs it would be “a matter of weeks” before COVID-19 testing problems were resolved.
He announced plans to ration tests, saying there would be “prioritisation” for people with acute clinical need in hospitals and those in social care settings.
Mr Hancock told MPs there had been a “sharp rise” in people coming forward for a test, including some who are not eligible, but Government sources acknowledged there was no accurate data on the latter.