Three patients have been killed by hospital sandwiches and 43 NHS trusts placed on alert after a deadly bacterial outbreak.In one of the worst cases of food poisoning ever seen in the health service, six people were diagnosed with a serious listeria infection between April 25 and May 15 and three have since died at Manchester Royal Infirmary and Aintree Hospital in Liverpool.Public Health England (PHE) and the NHS were under pressure last night to explain why it had taken so long to warn the public after they confirmed the deaths were linked in May.All those who died had been given sandwiches supplied by The Good Food Chain, a company based in Stone, Staffordshire, whose Whole Lotta Good brand was developed specifically for hospitals “to meet patients’ needs” and “deliver more for less”.The company, which also supplies some schools, has voluntarily ceased production while an investigation continues, as has the firm which supplied its meat, Northern Country Cooked Meats, which operates out of Salford. The Good Food Chain’s websiteCredit:The Good Food Chain A spokesman for North Country Cooked Meats said: “At this stage, the company is currently co-operating fully with the environmental health and the FSA in their investigations, and a further statement will be issued in due course.”Although listeria infection is rare, only causing mild illness in healthy people, it can survive at refrigeration temperatures which means it can persist in food-processing equipment and in the food chain.All those who have died and became ill had serious underlying health conditions and some are believed to be pensioners. The Good Food Chain has also supplied several Premier League football teams, motor and horse racing events, universities and colleges.It boasts that customers have “peace of mind” that all of its products are “lovingly hand prepared in a safe environment, as you would in your own kitchen”.Martyn Corfield, 50, founded the company in 1996 after a career in IT. According to Companies House, The Good Food Chain Limited is ultimately controlled by Mr Corfield.He is listed as a shareholder along with Atalanta Motor Cars Ltd, a classic car company which he also runs. In its mission statement, North Country Quality Foods says that it “is dedicated to producing and supplying the highest quality products, precisely to our customers needs”. The death of a third patient, who became ill on May 15, was also found to be caused by the same strain and by May 24 staff had raised suspicions that the sandwiches could be to blame.An immediate investigation was launched by PHE and The Food Standards Agency (FSA) and sandwiches and salads withdrawn from sale the following day.By June 2, preliminary tests suggested that the strain had originated from food supplied by The Good Food Chain. The results were confirmed on June 5. But the public was not informed until last night. Forty-three of the 135 NHS trusts have been placed on alert for cases of listeriosis, which can lead to deadly sepsis and meningitis. In pregnant women it can also cause miscarriage and be passed on to the unborn child.Dr Kimon Andreas Karatzas, associate professor in food microbiology, at the University of Reading, said: “Listeria monocytogenes is the most deadly food-borne bacteria. Due to a relatively long incubation period it’s possible that there could potentially be some more cases.”The first victim fell ill on April 25 and a second at the beginning of May. When both patients died, DNA testing was carried out which confirmed the same strain of bacteria had killed both. “To date, there have been no associated cases identified outside healthcare organisations, and any risk to the public is low.”The three patients who did not die are from trusts in England that have not been named. There are no cases in Scotland or Wales.Dr Colin Sullivan, chief operating officer at the Food Standards Agency, which is helping with the investigation, said: “We have taken action along with local authorities to minimise the risk based on the evidence so far.“The FSA will continue to investigate how the outbreak occurred and if further steps are required to protect vulnerable groups.” Show more Dr Nick Phin, deputy director at the national infection service at PHE, said: “Our thoughts are with the families of those patients who have died. However, there are fears that more people may be infected because of the long incubation period of listeria. The bacteria can lie dormant for up to 70 days and sandwiches were not withdrawn from the NHS until May 25 after investigators realised they had been eaten by all three of the victims. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.