Fewer diabetes patients in Heywood, Middleton and Rochdale received important annual health checks in 2021 than before the coronavirus pandemic, new figures show.
Diabetes UK said missing health checks can be "absolutely devastating", potentially leading to life-altering complications and early death.
The concerning figures come as Diabetes Awareness Week marks an annual campaign to inform the almost 5 million people with diabetes in the UK about the health risks it can cause.
To help limit the impact of diabetes, patients are expected to undergo eight annual health checks, including analysing their body mass index, smoking status, blood pressure and cholesterol.
NHS Digital figures show just 28% of 255 type 1 diabetes patients registered with GPs in the NHS Heywood, Middleton and Rochdale CCG area received all eight health checks in 2021 – down from 37% in 2019.
For the 5,620 type 2 patients in the region, this figure rose to 37%, but was still down on pre-pandemic levels of 51%.
Nationally, just 26% of type 1 and 39% of type 2 patients completed the eight checks – respectively down from 31% and 50% in 2019.
An NHS-commissioned report published this year said more than 3,000 people with diabetes in England died during the pandemic due to the drastic drop in patients receiving their annual check-ups.
The study measured a 15-week period in summer 2021 against the same period in 2019.
One of the authors of the report, Jonathan Valabhji, the NHS's national clinical director for diabetes and obesity, said the research "highlights the importance of annual reviews and ongoing supported management for people living with diabetes to manage their condition well".
Diabetes UK said the declining proportion of patients receiving necessary checks is hugely concerning and called on the Government to address the growing backlog caused by successive lockdowns.
Helen Kirrane, head of policy, campaigns and mobilisation at the charity, said: "While the UK Government was focused on cutting waiting lists for operations and other planned care, people with diabetes were pushed to the back of the queue.
"Urgent action is now required, which is why we’re calling on the UK Government to prioritise the recovery of diabetes care and to recommit to improving outcomes for people with and at risk of diabetes."
The Department for Health and Social Care said getting diabetes patients back into routine care is "essential for effective treatment and management".
A spokesperson said: "The NHS is supporting local systems with £36 million in funding to help them increase uptake and tackle health inequalities, and last year, the number of diabetes patients receiving all eight care processes increased by nearly a third compared to the year before."
Alongside the eight health care checks, patients with diabetes are given haemoglobin, blood pressure and cholesterol targets to reach each year to reduce the chance of complications.
Despite the fall in people receiving all eight check-ups, the percentage of people with type 1 diabetes in Heywood, Middleton and Rochdale meeting three key thresholds has risen – 20% of patients reached all three targets last year, up from 18% in 2019.
But for those with type 2 diabetes, 39% of patients achieved all three health aims, compared to 42% before the pandemic.