Almost 400,000 people in the Westcountry will have diabetes by 2020, a national charity has warned.
Over the next eight years the number of people with the condition in the South West is expected to rise by 64,000, according to a report by Diabetes UK.
Currently some 335,000 people in the region are diagnosed with diabetes, and that figure is projected to reach 399,000 by the end of the decade.
In Devon the number of diabetics is expected to increase by 10,800 to 63,300, including an increase of 2,200 to 17,100 in Plymouth.
In Cornwall and the Isles of Scilly numbers are expected to reach 45,900, an increase of 7,700.
Dr Kerry Bailey, a public health specialist in Cornwall, said: "There is a slightly higher rate of incidence in Cornwall because of the ageing population and levels of deprivation.
"The awful predictions are a warning of what could happen unless we do something. There needs to be a focus on prevention rather than waiting for the problem to get worse.
"Some people need to become more active, lose weight, and bring their sugar levels back to normal."
She added: "Evidence shows large numbers of people are unaware they have Type 2 diabetes. But people can assess their level of risk online, or visit their GP for a test and a health check."
Numbers of people with diabetes in Britain is expected to hit 4.4 million by 2020, a rise of 700,000, based on the data from Yorkshire & Humber Public Health Observatory. Many of the extra 700,000 cases will be Type 2 diabetes, which is often linked to lifestyle and weight.
Rising numbers will also be affected by Type 1 diabetes, which scientists are unable to explain.
Diabetes UK has warned that the cost of providing healthcare for the extra 700,000 people would put great financial pressure on the NHS, which is already spending 10% of its entire budget on treating the condition.
The charity is calling for the Government to fund a public health campaign to raise awareness of risk factors and of the seriousness of diabetes.
It also wants the Government to give more support to people at high risk to help them lose weight through healthy eating and regular physical activity.
Graham Cooper, Diabetes UK South West regional manager, said: "The healthcare system is already at breaking point in terms of its ability to provide care for people with diabetes and the result is that many people are developing health complications that could have been avoided and are dying early as a result.
"If this projected increase becomes reality, it would be a calamity for the healthcare system and a disaster for public health. But the Government and the NHS do not seem to have grasped the scale of the impending crisis and at the moment we seem to be sleepwalking towards it.
"We still hear about people who think diabetes is a relatively mild condition and do not realise it can lead to devastating complications such as blindness, amputation and stroke."