Staff absence in the Sussex University Hospitals Trust reached its highest point since the coronavirus pandemic began in December 2021, new figures show.
The Royal College of Nursing said overstretched workers are at breaking point, with many experienced nurses leaving the profession, while the British Medical Association has called on the Government to "focus on the protection and wellbeing" of the NHS workforce.
NHS Digital figures show 5.58% of the available full-time equivalent working days were classified as absent in the University Hospitals Sussex NHS Foundation Trust – the highest since the pandemic reached England in March 2020.
Further figures, also from NHS Digital, show that 5.96% of nurses and health visitors, 1.47% of doctors and 5.68% of midwives were absent.
Across England, NHS staff absences soared to 6.17%, the second-highest mark since records began in 2009 and only marginally behind April 2020 (6.2%).
Anxiety, stress, depression and other mental health reasons were the primary factor, accounting for 23.7% of all staff absences.
They were followed by coughs and colds at 12.7% and infectious diseases at 9.5%.
In the Sussex University Hospitals Trust, 2,822.44 full-time equivalent days were lost due to Covid-19, separate NHS Digital figures show.
RCN England Director Patricia Marquis said nursing staff have been pushed to breaking point and deserve better treatment.
"These services cannot afford to lose loyal, hard-working professionals to avoidable illnesses on top of tens of thousands of nursing vacancies," said Ms Marquis.
She urged employers to work with nursing staff so they can better care for patients and prevent many from missing work or even leaving the profession entirely.
Dr David Wrigley, council deputy chair at the BMA, said that the NHS faces an unprecedented staffing crisis without government action.
The BMA called for greater Covid-19 testing, more PPE provision and more targeted public health measures across the country during the final quarter of last year, when staff absences reached their highest point since records began.
"We need a focus on the protection and wellbeing of NHS workers, with a long-term national workforce strategy to grow the workforce and retain the experienced staff we have," added Dr Wrigley.
The Department for Health and Social Care said it is supporting the health and wellbeing of NHS staff by providing targeted psychological support and treatment, occupational health sessions, and investment in "wellbeing conversations".
"We have record numbers of staff working in the NHS, including over 4,200 more doctors and over 12,100 more nurses compared to January 2022," a spokesperson said.
The DHSC established the People Recovery Task Force as part of the Elective Recovery programme and invested £37 million in 2021-22 in 40 mental health hubs across the country.
The NHS will also develop a long-term workforce strategy, the spokesperson added.