Drug watchdogs have been criticised for proposing restrictions on who gets a modern diabetes drug.
The drugs will be available to some
The National Institute for Clinical Excellence says that patients should only get glitazones when cheaper treatments have failed.
The drug could help control the condition, and stop the emergence of devastating diabetic complications.
The charity Diabetes UK says that doctors should be allowed to prescribe the drugs they think are needed.
Nice, an independent body which advises government on which drugs are cost-effective, decided that new research evidence did not justify the use of the drugs in preference to existing treatments.
It predicts that the number of people receiving the drugs could fall by approximately 30% - or 50,000 people a year.
The move will save the NHS money, said a spokesman.
Instead, patients will be given, if anything, other drugs such as metformin or sulphonylurea to control their condition.
There are an estimated one million people in the UK diagnosed with type II diabetes, which gradually reduces the patient's ability to control blood sugar levels.
If left unchecked, it could lead to complications such as eye damage, ulceration or even lower limb amputation.
Glitazones worked by increasing the effectiveness of the hormone the body uses to control blood sugar - insulin - and by preserving cells in the pancreas which make it.
They are potentially an alternative to insulin injections for some patients.
Suzanne Lucas, director of care at Diabetes UK, said that the decision was wrong.
She said: "Doctors and patients need to have access to the treatments that are best for them. Today's ruling will deny that.
"Diabetes is costing the NHS millions of pounds a day, much of which is spent on treating the long-term effects of the condition, many of which could be prevented with early, effective treatment.
"Nice needs to look at the long term future rather than base decisions on trying to make small financial savings today."
Nice also came under fire for a second decision published on Wednesday.
While it decided to support doctors who wanted to prescribe human growth hormone to adults, the Society for Endocrinology said it felt the guidance was "far too restrictive".
Professor Michael Sheppard, the society's chairman, said: "I remain concerned that there are many patients who would clearly benefit from growth hormone replacement therapy who will not have access to NHS funded treatment."