Three toddlers remained seriously ill in hospital last night with kidney failure following a suspected outbreak of E. coli.

The three two-year-olds, who attend a nursery in Dunfermline, Fife, were being treated at Glasgow's Yorkhill Hospital.

The children are believed to have been struck with the O157 strain, the same as the one that killed 21 people in Wishaw, Lanarkshire, in 1996.

The private nursery was closed yesterday while health chiefs investigated the cause of the outbreak.

NHS Fife said the first child became ill on Friday, while the other two cases came to light on Monday and yesterday.

Last night, all of the children remained seriously ill, although one had shown a slight improvement, said a trust spokeswoman.

Dr Charles Saunders, consultant in public health medicine, said the source of the infections was not yet known.

"We are writing to all parents and staff at the nursery with advice on E. coli O157 infection and offering screening for E. coli O157 infection," he said.

"We have not so far been able to exclude possible food or water sources.

"We are working with Fife Council's environmental services department. It is certainly possible that other children who attend the nursery may be affected."

A spokeswoman for the nursery, which is owned and run by the Careshare Group, said there was no evidence to indicate the nursery was the source of the illness, but it had been shut as a precaution.

"We reported the illness immediately to the authorities, who have inspected the nursery and our hygiene procedures, and have confirmed that they are satisfied," she said.

The Food Standards Agency Scotland said it had been made aware of the investigation and was being kept informed.

The incubation period for E. coli O157 is usually between one and 14 days. Symptoms can include diarrhoea, feeling sick, vomiting, stomach pain and fever.