Nearly half of diabetes patients in Wales are not getting the essential annual health checks they need, according to Diabetes UK Cymru.
To mark World Diabetes Day, the charity says a UK survey shows a third of people have not been offered any education on managing the condition.
One in four people are not getting annual foot checks and 20% do not have annual kidney function tests, it says.
It has published a list of what it calls 15 essential healthcare checks.
Diabetes UK Cymru said 5% of Wales' population had diabetes - compared with a UK average of 4.5% - and the overall level rose by 4.4% in the past year.
The highest incidence is in Blaenau Gwent with 6.47%, and the lowest Cardiff with 3.6%.
Diabetes UK surveyed 7,000 patients across the UK.
It said the survey reinforced Welsh figures from the National Diabetes Audit that 43% of people had not had all the essential health checks.
At least 27,000 people may not be getting an annual foot exam, and therefore could be at risk of amputation.
It added that people with diabetes in Wales were five times more likely to have a minor amputation and 2.5 times more likely to have a major amputation than the general population.
Diabetes UK Cymru director Dai Williams said: "There are many diabetic complications but in large part they can be prevented if people regularly receive the care and support they need to manage their condition.
"Denying people the chance to live normal, healthy lives by not providing them with the care they need is an outrage.
"It is vital that every single one of the 160,500 people with diabetes in Wales gets the 15 healthcare essentials they need.
"We call on them to claim the care they are entitled to and we call on the many healthcare professionals who are already backing the campaign to ensure this becomes a reality for their patients."
Last month, the charity called for "a massive awareness campaign as a matter of urgency" to raise awareness about Type 2 diabetes.
It said the disease was costing the NHS in Wales £500m each year.
At the time, the Welsh government said it would continue to invest in raising awareness of the importance of a healthy lifestyle, and had established an "expert group" to see what further work was needed.