A food poisoning alert has been issued over ready-made sandwiches sold through hospitals, schools, council sports centres and canteens.

The Food Standards Agency raised the alarm yesterday, after a very small number of samples tested positive for listeria during routine checks.

However, the suspect sandwiches have already been eaten because they were sold during a three-week period in February and March.

Listeria is known to be a risk for pregnant women, the elderly and sick, who tend to have a weaker immune system.

Symptoms of infection can take up to 90 days to appear after exposure and may start with a fever, flu-like illness and diarrhoea.

Anyone in the vulnerable groups who may be worried that they have been exposed and develops these symptoms should take advice from their GP or hospital consultant.

The illness can usually be effectively treated with antibiotics. The Health Protection Agency is not aware of any cases of infection but it has sent an alert to GPs in London and the South East, where the sandwiches were sold.

As many as 220,000 produced between February 21 and March 14 are under suspicion. They were manufactured by Anchor Catering, based in Ashford in Kent, and distributed in Kent, Sussex, Essex, Surrey and Greater London.

Among the hospitals involved are Guy's and Great Ormond Street in London, plus others in Kent and Sussex.

The schools include Tiffin Girls at Kingston upon Thames and Christ's College, Guildford. In Kent, Canterbury Cathedral, Hever Castle, Leeds Castle and Dover Castle are also affected.

The majority of the sandwiches were labelled as Anchor products. Others were sold under the Pomegranate name. The FSA has not identified the type of fillings involved.

Dr Graham Bickler, South East regional director of the HPA, said: "We are not aware of any infection at present but want people in those vulnerable groups to be aware that if they have eaten sandwiches from this company-during the period in question and become ill within two to three months they must seek medical advice."

The FSA is working with Anchor Catering to identify the source of the contamination. Until this is done, production has been suspended.

The agency's chief scientist, Dr Andrew Wadge, said: "Although the company took prompt action to notify customers and withdraw the product, large numbers of sandwiches may have been consumed before the problem was detected."