Results from an ongoing study by the University of Nantes in France also showed that one in eight diabetes patients admitted to hospital with coronavirus were still there 28 days later.
Diabetes UK said understanding which people with the condition are at a higher risk if they are admitted to hospital with Covid-19 will help to improve care and save lives.
The findings show that within 28 days of being in hospital 577 of the 2,796 patients studied (21 per cent) had died.
Almost 50 per cent (1,404) had been discharged from hospital, with a typical stay of nine days.
Around 12 per cent remained in hospital at day 28, while 17 per cent had been transferred to a different facility to their initial hospital.
The authors of the study, published in the Diabetologia journal, said: “The identification of favourable variables associated with hospital discharge and unfavourable variables associated with death can lead to patient reclassification and help to use resources adequately according to individual patient profile.”
In May last year, earlier results from the study, based on smaller sample of people, suggested that 10 per cent of Covid patients with diabetes died within seven days of a hospital admission.
Dr Faye Riley, senior research communications officer at Diabetes UK, said the study supports previous research which showed certain risk factors, such as older age and a history of diabetes complications, “put people with diabetes at higher risk of harm if they catch coronavirus”.
“It also provides fresh insight into factors that are linked with a quicker recovery from the virus,” she said.
Dr Riley said: “Understanding which people with diabetes are at a higher risk if they’re admitted to hospital with coronavirus will help to improve care and save lives.
“But it’s also important to remember the overall risk of dying for people with diabetes remains low, and has reduced over the past year.
“Since the data in the study was captured, our understanding of how to treat coronavirus has grown, and new drugs shown to reduce risk of death are now in routine use.
“The best way people living with diabetes can lower their risk of becoming seriously ill from coronavirus is to avoid contact with the virus and take a vaccine.
“In the UK, people with diabetes are now being invited for vaccination, with those at highest risk being prioritised and invited to shield, and we strongly encourage you to get the vaccine when you’re offered it.”