Some 8.6 million people threw sick days last year because they found their jobs "too painful", a survey suggests.
Released on what some dub "National Sickie Day", the research claimed concerns about work culture, colleagues and workloads were to blame.
However, it also said 12 million workers went to work genuinely sick.
The IT company Insight, which did the research, warned of "serious issues within organisations' culture" and called for more flexible working.
It based the findings on a Kantar survey of 1,246 working adults, done over a week in January this year.
The responses were weighted to draw a picture of the wider working population, which numbers almost 33 million people, according to official figures.
"Employers have a duty of care to their employees to look after their safety and wellbeing, and this includes their physical and mental health," said Tom Neil, Acas Senior Adviser.
"For people to be able to be honest about how they feel at work, good work practices including having an inclusive culture and effective people management are key."
In the survey, a quarter of respondents said they had taken a sick day in the last year because doing so felt "too painful".
Some said they felt overworked or that poor systems and processes made it hard to get work done. Others blamed conflicts with workmates.
However, 37% of respondents said they had come into work in the past year despite feeling sick.
Many said this was because they could not afford unpaid sick leave or did not want to use up a paid sick day. Others said they did not want to feel judged by their employer or co-workers.
Meanwhile about a fifth - or an estimated 6.5 million - said they would be happy to work from home when sick but their firms would not let them.
According to some surveys, more employees call in sick on the first Monday of February than any other day of the year, with an estimated 215,000 doing so last year.
But in 2019, the employment law firm Elas said it had found other days with higher absence rates.
It said the top 10 days for absence last year all fell on a Monday, with 16 September topping the list. The most comment reasons given were:
- Stomach trouble (24%)
- Cold, cough and flu (16%)
- Headache (7%)
According to the Chartered Institute of Personnel and Development, the typical employee's number of sick days dropped to 5.9 in 2019 - the lowest in the 19-year history of its annual survey of HR professionals.