National Diabetes Audit data also shows that just 22.4% of those who have had Type 2 diabetes for up to four years (thought to be about a million people) meet recommended levels for blood glucose, cholesterol and blood pressure.
Diabetes UK said it was concerning due to the increased risk of future complications such as kidney failure and amputation. It warned that better support should be provided to sufferers as soon as they are diagnosed.
There is evidence that unless Type 2 diabetes is controlled well at the start, the body adapts to having high glucose levels and the longer this goes on, the more difficult it is to get under control.
But the charity said that at the moment just 14% of people with Type 2 are offered diabetes education soon after being diagnosed, despite the National Institute for Health and Care Excellence recommending that this be offered to everyone with the condition.
Diabetes UK is calling for the Government and NHS to do more to ensure people get the support and education they need after being diagnosed to enable them to manage their condition.
The charity is holding a series of free educational events at 80 locations across the UK with the help of its National Charity Partnership with Tesco.
The Living with Diabetes Days will give people the chance to learn how to maintain a healthy diet and control their diabetes as well as ask health professionals questions and meet others with the condition.
Chief executive of Diabetes UK, Barbara Young, said: "With increasing evidence about the importance of getting Type 2 diabetes under control as quickly as possible, it is extremely worrying that just one in five people diagnosed with the condition in the last four years have it under control.
"This means many hundreds of thousands of people are at increased risk of developing health complications such as kidney failure and amputation, which have a devastating impact on people's lives and are fuelling the high death rate in people with the condition.
"And while it's important that everyone with diabetes has their condition under control, if people with Type 2 do not get it under control quickly then it becomes progressively harder to do this so speed is vital in terms of getting people the support they need.
"Unfortunately, a big part of the reason that so many people with Type 2 are starting off on the wrong path is the lack of available diabetes education. It is unrealistic to expect people to be able to manage their condition well if they are not given information about how to do this and so it is no surprise that so many people do not have it under control.
"This is why we want the NHS to give every person with diabetes the chance to have this kind of education."
Visit www.diabetes.org.uk/lwdd or call 0345 123 2399 for more information.