Specialists from Public Health England and environmental health officers are investigating the cases of E. coli O157, which are linked to Little Brook Childrens Nursery, in Great Harwood.
Testing is being carried out on staff and children who may have come into contact with the bacteria, and children have been asked to remain at home until they have tested negative.
A spokeswoman for Public Health England said that although staff were at the nursery yesterday, it will effectively be closed to children until those given the all clear begin to return.