Staff at Cornwall Partnership NHS Foundation Trust took more days off sick last December than in any month since the start of the pandemic.
The British Medical Association has warned the NHS is facing an "unprecedented crisis", with burnt-out staff unable to work and a lack of investment causing further stress, anxiety, and depression.
The latest NHS Digital figures show the NHS staff sickness absence rate was 6.3% in England in December 2022 – up from 5.4% the month before, and slightly higher than 6.2% a year previously.
At Cornwall Partnership Trust, 7% of the available full-time-equivalent working days were missed in December 2022 – the most of any month since at least March 2020.
One month earlier, the rate was 5.5%, while in December 2021 6% of all FTE working days were classed as absent.
Separate figures from NHS Digital show approximately 1,490 working days at Cornwall Partnership Trust were lost due to Covid-19.
Nationally, colds, coughs and flu were the most reported reason for sickness, accounting for over 580,000 FTE days lost and 22% of all sickness absence in December 2022. This increased by almost 10 percentage points since November.
Anxiety, stress, depression and other psychiatric illnesses were the second most reported reason, accounting for nearly 550,000 FTE days lost.
Clinical support staff lost the most working days with just over 1 million, followed by nurses and health visitors with 740,000, and infrastructure support staff, including NHS managers with 380,000.
Dr Latifa Patel, chair of the representative body and workforce lead at the BMA, said: "This Government needs to step up and ensure that doctors are properly supported for their own health with a system that can meet the needs of patients without adding to staff burnout.
"The NHS is in the worst state it has ever been in, yet there seems to be no plan in place to ensure that our health service is properly resourced, which is threatening patient safety and putting doctors’ health at serious risk."
The highest sickness absence rate in the country was in the North West, at 7.4%. London reported the lowest rate at 5.4%.
A Department of Health and Social Care spokesperson said a new workforce plan focusing on recruiting and retaining more staff will be published later this year.
They added: "We are hugely grateful to NHS staff for their hard work and their health and wellbeing is of paramount importance.
"For those staff that need it the NHS provides physical and mental health support – including targeted psychological support and treatment."