According to research from law firm GQ Littler, the number marks a 17% increase on the 8.8 million notes given out during 2020.
Coronavirus was seen as a major contributing factor, given the high number of cases after people returned to offices following lockdown.
There were 7.3 million reported Covid cases between 1 April and December 31 2021, compared to 2 million for the same period in 2020.
Covid's impact on the workplace:
Rising covid infections force HR to prepare
Long Covid impacting half of UK organisations
Living with Covid and the end of isolation: employers risk excluding vulnerable
Sophie Vanhegan, partner at GQ Littler said: "The spike in fit notes may reflect a rise in Covid infections following the end of lockdowns in 2021. As Covid cases start to rise again, employers are all too aware that this may hit staffing levels at a time when businesses are already under pressure from staff shortages."
"One legacy from the pandemic is it is now frowned upon to come into the office whilst visibly unwell. This could also have contributed to the increase in notes being issued in roles which cannot be performed from home."
Earlier this month government changed the legislation surrounding fit notes, so that medical professionals other than GPs - such as nurses, pharmacists and physiotherapists - could issue them.
The research also showed an increase in fit notes issued for stress-related reasons. The figure increased 10% from 257,000 in 2020 to 282,000 in 2021.
Vanhegan added: "Supporting employees through their mental health issues is extremely important, not just for productivity and morale. In today’s competitive jobs market, organisations that are viewed as unsupportive may have difficulty attracting and retaining talent."
