A gastroscope was used to examine patients

On Monday, it was announced that 3,000 patients were being contacted and told equipment used on them may not have been disinfected properly.

Health chiefs declined to give any more detail about how such problems had arisen.

However, the BBC has learned that four problems were found in 17 of the instruments at the Royal Victoria Hospital in Belfast, Antrim Area Hospital, Lagan Valley Hospital in Lisburn and Whiteabbey Hospital.

The equipment is used to detect problems in the throat and stomach, and has to be sterilised after each use.

The first problem concerned the use of incorrect connecting equipment in the decontamination process, involving parts which weren't compatible with the model of the endoscope.

GASTROSCOPES Small flexible tube with a light and camera at the end Passed down patients' throats Used to check for conditions like ulcers, and to take tissue samples Must be sterilised after every use Cleaning must be done carefully with chemicals, as microscopic infections could be present

The second was failure to clean all the channels in some scopes - even ones that had not been used in the last procedure.

The third shortcoming was failure to manually de-contaminate an auxiliary channel while the use of too low a level of disinfectant also caused a problem.

The Medicines and Healthcare Products Regulatory Agency have now asked hospitals in England to immediately carry out their own checks.

Over 1,300 patients who had undergone treatment at the Royal Victoria Hospital, and Whiteabbey Hospital, are being advised to have blood tests.

Patients, who had undergone the procedure in Lagan Valley Hospital, and Antrim Area Hospital, are being told they are at low risk and do not need the test.