Tens of thousands of women may have to re-take pregnancy tests after a manufacturer revealed that a small batch used in UK hospitals were faulty.
Home kits are not affected
It is believed up to 44 "Clearview HCG" tests could wrongly indicate that a woman is not pregnant.
Any woman who was given one of the tests in hospital has been asked to seek medical advice, or test themselves again using an over-the-counter kit.
The faulty tests were part of a batch of 83,000 sent to hospitals.
Urine tests are frequently given in hospital to rule out pregnancy before medical procedures, or to help detect ectopic pregnancy.
The faulty batch, made by US firm Inverness Medical Innovations and distributed in the UK by Unipath, could have been used between March and June this year.
The company has written to 50 hospitals in the UK that use the product, warning them of the possible "false negative" result.
The test is only supplied to hospitals, and similar tests made by Inverness Medical for sale at pharmacies are not affected.
Kingston Hospital in London took the decision to alert 1,400 women tested during that period.
In a statement, it asked any woman with concerns to contact the clinic or consultant where they were seen.
The Medicines and Healthcare Products Regulatory Agency, which oversees the use of "medical devices" in hospitals, confirmed that the recall had taken place.
A spokesman said: "When using tests from this batch of product on samples from patients who are pregnant, in a very small number of cases the result may be either negative or invalid."
The Royal College of Obstetricians and Gynaecologists said that women who are worried that they may have received a faulty test should contact their GP or hospital, or buy a second test from a pharmacist to double-check.
A spokesman for Unipath said that the fault went undetected because such a tiny proportion of the 83,000 in the batch were affected.
She added: "We are confident that the problem has been correctly diagnosed and is now being rectified by changes to the manufacturing process.
"We would like to apologise for any inconvenience this recall may have caused to our customers or their patients."