Jewish diabetics have been warned to consult their doctors and their rabbis before they embark on the Yom Kippur fast.
Diabetes UK said prolonged fasting could cause sharp falls or rises in the levels of blood sugar glucose.
"This can be very dangerous for people with diabetes, as can the increased risk of becoming dehydrated," said the charity's clinical adviser, Libby Dowling.
She urged sufferers to talk to their doctors about how to fast safely, and alert their rabbis so they were aware of the potential dangers for diabetic congregants.
Ms Dowling said Jewish people with the condition had to exercise more caution during the "feasting and fasting" of the High Holy Days.
"Having diabetes doesn't need to stop people from eating traditional festive foods, and that even sweet foods, honey and alcohol can be enjoyed, but this needs to be in moderation," she said.
"The key is to plan and be aware of how a treat may affect your blood glucose levels."