The startling verdict is the result of a survey of almost 2,000 adults which found that age discrimination is the most common form of prejudice, with 11% of workers believing they had been treated unfairly as a result of their age.
The figure is 3% more than those who felt they had been discriminated against because of their gender.
Age and sex were also pinpointed as the most prevalent forms of workplace harassment.
Gillian Dowling, technical consultant at business consultancy Croner, warned employers to expect a rise in tribunal claims related to age discrimination unless they stopped the practice.
Ms Dowling said: “This form of prejudice has quickly become one of the more prominent forms of workplace discrimination defined by employment law.
“We are advising employers to ensure that “too young” or “too old” is no longer a factor in any employment decision-making, such as hiring and firing.
“They should also be aware of any unfair treatment of their employees.”