One in four workers would have to be in hospital before taking a day off sick
A survey of 1,000 employees by insurance group Canada Life found that nine out of 10 had gone to work while feeling ill, unchanged from similar research a year ago.
The report said employers' efforts to improve the wellbeing of employees are failing to reduce a culture of "presenteeism".
Just over half of those polled said they would go into work if they had the flu despite the chance of spreading the virus to colleagues.
Many of those who do not take time off when they are ill blame high workloads or financial concerns.
Paul Avis, marketing director at Canada Life Group Insurance, said: "It is incredibly worrying it would take something as serious as being hospitalised to dissuade a quarter of British employees from going into work, showing that a 'stiff upper lip' culture of presenteeism still pervades the British workforce.
"People suffering from illnesses like flu and stomach bugs are unlikely to be productive and risk making their colleagues unwell as well by struggling into work.
"We need to be clearer with employees - they should only come in to work when fully fit and able to do so, be it physically or mentally."