Vulnerable people are being warned not to eat under-cooked or runny eggs when they are eating out because of the developing salmonella outbreak.

The Food Standards Agency (FSA) told the BBC the under fives, over 65s, women who are pregnant and those with health conditions should avoid them along with freshly-made products that might contain uncooked eggs, such as mayonnaise, souffle and hollandaise sauce.

But the FSA said well-cooked eggs in restaurants, cafes and from takeaways were fine to eat.

It comes after the UK Health Security Agency declared a national outbreak of salmonella food poisoning after one person died and hundreds more fell ill.

It is thought imported eggs or dishes containing them are behind the outbreak.

FSA chief scientific adviser Ian Young said: “If people are eating out or consuming eggs or egg-containing products from cafes and restaurants then for those people in particular who are young, elderly, pregnant or vulnerable, it’s important to make sure that any eggs or egg products that you consume in those settings have been very well cooked.”

But he reiterated the message that it was fine for people to buy British eggs from supermarkets as chickens bred here are vaccinated against common strains of salmonella.

“There is no link to those eggs to this current outbreak.”

He described the outbreak as “unusually large and rapidly increasing”.

“People need to be careful,” he added.

There have been over 200 cases in the UK linked to this outbreak. The majority are across England.

Genetic testing suggests all the infections are part of the same outbreak and there are similarities with previous outbreaks involving imported eggs.

Investigations into individual cases also suggest the eateries involved were buying eggs from abroad.

Salmonella are a family of bacteria that typically live harmlessly in the digestive tract of animals including cattle, pigs and chicken – which is why eggs and poultry are among foods commonly associated with the infection.

People can catch it by eating contaminated food that has not been cooked properly, or cross contamination after mixing raw and cooked food in the kitchen or coming into contact with infected animals.

It can then cause stomach cramps, diarrhoea, vomiting or fever. In most cases people recover without further help but older people, babies and people with weakened immune systems are at greatest risk of severe disease.

Hospital care with fluids and sometimes antibiotics can help in these cases.

Anyone who is concerned is being advised to contact their GP or out of hours service in the first instance.