Women who were given a negative pregnancy test result on the NHS this spring were last night urged to contact their doctor after a faulty batch of pregnancy testing kits was identified. Dozens of women could be months into a pregnancy without knowing it, the Medicines and Healthcare products Regulatory Agency (MHRA) warned.

Health campaigners said many of these women may have unwittingly had medical treatments, consumed alcohol or taken drugs which could be harmful to their child. There were further concerns that some would find themselves months into an unwanted pregnancy with no option but to continue.

The makers of the Clearview HCG tests, Unipath, said that about 44 of the 83,000 kits it supplied to 50 hospitals were affected. No faults were found with their home-testing kits sold in chemists and women who were told they are pregnant were not affected.

The search for the 44 recipients of the faulty tests was described by health officials as looking for the "needle in the haystack". The MHRA said that a small number of the tests had also been used in clinics and at GP surgeries and all women who had been given a negative result in March, April or May should contact their GP or hospital.

Vanessa Bourne, of the Patients Association, said: "These tests are very sensitive very early on in pregnancy in order to protect women and their babies. That's why it's so important that we have absolute confidence in them."

Colin Todd, medical director for Kingston Hospital NHS Trust which gave 1,400 patients a Clearview test between April 12 and June 1, said: "As a precaution, the hospital wishes to alert patients and if they have concerns to contact the clinic or consultant where they were seen."

Pregnancy tests are given to thousands of women every month before having any treatment which is contra-indicated for pregnant women.

A spokesman for the Royal College of Obstetricians and Gynaecologists, said: "Any concerned woman should go back to their hospital or see a GP. They could also purchase a pregnancy test kit at a chemist."

Unipath, a division of Inverness Medical Innovations, said: "Clearview is only supplied to healthcare professionals for their use and is not supplied direct to consumers. An internal investigation has indicated that the fault was caused by one strip of material that was incorporated into 44 tests.

"This fault was not detected because of its extremely low incidence - the 44 faulty tests were part of a batch of 83,000 tests. Unipath has sent recall documents to all the organisations that may have received this batch of product with information about how to return or destroy these tests."

The MHRA said in a statement: "The affected lot was distributed to UK hospitals between March and April this year. 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 manufacturer has written to their customers to inform them of the problem and advised that products from this lot should not be used. MHRA will be publishing the manufacturer's recall letters and plan to issue further advice to the health service."