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.”

He described the outbreak as “unusually large and rapidly increasing”, and added that “people need to be careful”.

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 added.

Mark Williams, chief executive of the British Egg Industry Council, said eggs from Britain were safe to eat when runny, even for vulnerable people, and were widely available in restaurants and cafés.

“Customers who want to enjoy a runny egg should simply ask whether British Lion eggs are being used,” he said.