Doctors are being told to "think carefully" before ordering any tests for their patients, amid shortages caused by a supply chain failure at a major diagnostics company.
Swiss pharmaceutical firm Roche said problems with a move to a new warehouse had led to a "very significant" drop in its processing capacity.
A spokesman said COVID-19 tests would be prioritised, but the backlog could affect tests including for cancer and heart disease.
One NHS trust in the south west has already advised its GPs to stop all non-urgent blood tests.
A memo seen by the BBC, sent to clinicians within a large hospital trust in London, said leaders were "preparing for a sustained disruption".
"We urgently need all clinical teams to only send tests that are absolutely essential for immediate patient care, delaying testing where possible," it said.
Thyroid and cortisol tests were unavailable, while certain cholesterol, liver function and inflammation tests were "severely restricted".
Source: BBC News, 7 October 2020