Nearly forty people were in hospital today and a further 50 are believed to be affected by a mass outbreak of food poisoning in Bradford.

Some of the patients, who are being treated at Bradford Royal Infirmary, are in a "poorly" condition suffering from dehydration caused by severe sickness and diarrhoea.

An inquiry has started by the Health Protection Agency and Bradford Council environmental health officers to pinpoint the source of the outbreak of the gastro-intestinal infection which is thought to be a particularly virulent strain of salmonella.

One of the victims, Umar Mustapha Mahmood, 17, of Barkerend, Bradford, was admitted to BRI yesterday.

His father Tariq Mahmood said Umar had been out on Saturday night and had been feeling unwell ever since.

"He was vomiting and had acute diarrhoea. He had aches and pains, and stomach cramps and he was dehydrated," said Mr Mahmood.

"The hospital performed some tests on him to see what it was, and they said he had had food poisoning. He is really suffering, he is in agony."

Dr Dee Kyle, of Bradford Health Protection Unit, urged anyone who had similar symptoms to see their GP. It appears that this illness is caused by an organism which is known to cause food poisoning," she said.

"As with all cases of gastro-intestinal infection scrupulous hand-washing, particularly when preparing food or after using the toilet, is key to the prevention of spread.

"Patients experiencing frequent and large amounts of diarrhoea, with or without a high temperature or vomiting, are advised to contact their general practitioner for help and advice."

A spokesman for Bradford Teaching Hospitals NHS Trust, which runs BRI, said the scale of the outbreak had caused it to implement a contingency plan.

"Bradford Teaching Hospitals NHS Trust has admitted 37 people suffering from and diarrhoea vomiting symptoms," he said.

"The Trust has contingency plans for such circumstances but, at a traditionally busy time, the illness is placing considerable additional strain on Trust resources.

"Staff are working extremely hard and we are monitoring the situation very closely to minimize any potential impact on our normal services. We are also working closely with our colleagues in primary care and public health.

"Symptoms vary but some patients are suffering from dehydration and are in a poorly condition."