A nationwide alert was issued yesterday by the medicines regulator over a widely used pregnancy test that gives a wrong result on almost one in three occasions.

Thousands of women have received a negative result from the test, and an unknown number of these may still be pregnant. Doctors warned yesterday that the women could be putting their unborn babies at risk if they are receiving treatment or taking drugs which could be dangerous in pregnancy.

The Medicines and Healthcare Products Regulatory Agency (MHRA) said in a statement yesterday that checks on the SAS 1 Step testing kit, made by the US company SA Scientific, had revealed an unacceptable level of error. The University Hospital in Durham, which first identified the problem, said the error rate was up to 30 per cent, compared with a normal error rate for testing kits of less than 1 per cent. Sue Ludgate, medical director of the MHRA, said: "We will be issuing urgent advice on Thursday about the recall of this product. Clinicians and health professionals should not use this pregnancy testing kit and should retest those patients who may be adversely affected by a falsely negative result."

The test is used in hospitals, GP surgeries and clinics but is not sold over the counter. Women who bought a test for use at home are not affected.

The problem came to light during routine checks by the University Hospital, which revealed a faulty batch of the tests used on more than 3,000 women in the local health authority area between 1 October 2003 and 29 January 2004. A test called Clearview was introduced since 29 January. A spokesman for the hospital said 1,200 women had tested negative with the SAS 1 Step and 560 had been informed of the problem.

A spokesman for SA Scientific, based in San Antonio, Texas, said it was conducting tests to establish the cause.