Children with diabetes are at increased risk from ill health because of a lack of specialist nurses in London, a health charity has said.
Diabetes UK's report found some of the city's specialist diabetes nurses care for more than 100 children rather than the recommended 70.
The charity said it meant nurses did not have enough time with each child.
The report showed the workload for nurses in more than 40% of Primary Care Trusts (PCTs) had risen since 2006.
Phillip Watson, London regional Manager for Diabetes UK, said: "PCTs have to wake up and understand the vital role of specialists nurses.
"With some nurses coping with a heavy case load, it is no wonder so many children have poor blood glucose control. Most are struggling to even see a specialist nurse, so any additional support is out of the question.
"Services must improve now otherwise our children risk losing their sight or needing kidney dialysis in later life."
He said the government had promised to improve specialist care five years ago but the changes had not yet been made.
A Department of Health statement said: "The government has recognised that the quality of diabetes care for children and young people can be variable, and in October 2005 set up a working group to establish what needed to be done to improve this.
"In April 2007 this group published a report of their findings.
"The group's recommendations are now being taken forward by an implementation group led by Dr Sheila Shribman, National Clinical Director for Children and including representatives from Diabetes UK, Royal Colleges, the Healthcare Commission and other partner organisations committed to facilitating the implementation of best practice nationally."