Diabetes diet courses for only 2% of new patients in Wales, says Diabetes UK Cymru
Only 2% of the 7,000 people in Wales diagnosed with diabetes every year are offered the dietary courses the law requires, claims a diabetic charity.
Diabetes UK Cymru alleged the Welsh NHS was the "worst in the UK" at alerting people to the scale of the disease and its possibly fatal complications.
It says that though 160,000 people have been diagnosed with the "miserable disease," a further 350,000 have not.
The Welsh government said it continued to promote a healthy lifestyle.
Diabetes UK suggests that 80% of the UK NHS's £9.8bn annual diabetes bill goes on the cost of treating complications, and that by 2035 diabetes will take up 17% of the NHS's entire budget.
Dai Williams, director of the charity's Welsh arm, said: "It's the biggest threat we've got to the NHS.
"These kind of costs are totally unsustainable. My worry is that it could bring the NHS down, and the silly thing is, it's all largely preventable and that's the crying shame."
Mr Williams told BBC Radio Wales that he was amazed by the "inactivity" of the Welsh government in tackling the condition and informing those diagnosed with it how to prevent complications.
He said: "What it needs to do is make the public aware of the scale of the problem, that diabetes is indeed dangerous.
"It did it for swine flu - swine flu didn't happen luckily - it can do it for diabetes, which is with us at the moment.
"It's done nothing on the awareness front at all. That, I think, is incredibly wrong, essentially."
Mr Williams said National Institute for Health and Clinical Excellence (Nice) guidelines required newly-diagnosed diabetes patients to be provided with structured diabetes education.
But in Wales, this was offered to only 2% of such patients, he said.
He said: "To be reaching 2% of a legally obliging figure is scandalous. They [the Welsh government] need to be taken to task about that. We're the worst in the UK."
A Welsh government spokesman said: "The rise in diabetes can be attributed to many health-related factors and prevention is widely acknowledged to be better than treatment.
"Our aim is to encourage people from a young age to develop good lifestyle habits that they can take into adulthood.
"We are continuing to invest in raising awareness of the importance of a healthy lifestyle, including a healthy diet, drinking alcohol sensibly and not smoking, through creating environments that help people to make healthy choices.
"The provision of structured patient education to promote effective self-management of diabetes to keep people with this condition healthy and out of hospital is the responsibility of local health boards.
"Effective self-care will be a key theme in the National Diabetes Delivery Plan for the NHS for the period up to 2016 which will be developed over the coming months."