Four people have been diagnosed with sepsis after eating at a luxury restaurant in Stockholm as part of a suspected listeria outbreak.

Health authorities say at least 15 people have been treated or are under observation in hospital after dining at Coco & Carmen, a high-end venue in the Swedish capital's Östermalm district.

Most of the dozens affected are not seriously ill, officials in Stockholm say, and so far none of those admitted to hospital have required intensive care.

It is rare for food contaminated by listeria, a bacterium, to cause a very serious infection, known as listeriosis. People with listeriosis can develop sepsis - a life-threatening overreaction by the immune system.

The regional authority confirmed that infection tracing, initially covering guests who ate at the restaurant between 25 and 27 September, has since been widened to include people who dined there from 23 and 24 September.

Those with ongoing symptoms such as fever, vomiting or diarrhoea are now recommended to visit a hospital emergency room for assessment, Maria Rotzén Östlund, an infectious disease physician, told state broadcaster SVT.

Regional health officials warned in a press release that a large number of people may have been infected and emphasised that listeria posed the greatest risk to pregnant women, older people, and those with weakened immune systems.

SVT reports that 80 people have registered having food poisoning after dining at the restaurant, and 400 people have been contacted for infection tracing.

Coco & Carmen has temporarily closed while an investigation continues. The restaurant stated that it has always passed food safety inspections and is cooperating with authorities.

We take this extremely seriously and assure our guests that their well-being is our top priority. We have voluntarily closed until further notice, the owners said, adding that they hoped to reopen on 7 October.

Joakim Almquist, the chef and co-owner, expressed his regret saying, The only thing I can do is sincerely apologise, because I suffer with these people.

Listeria is commonly found in soil, water, and animals, and can spread through contaminated food, particularly when stored in cold conditions for long periods. While most experience only mild symptoms resembling stomach flu, it can lead to severe illness such as meningitis or sepsis. The incubation period can extend from several days to three weeks.