Workplace rights, including protection from unfair dismissal and the right to statutory redundancy pay, only apply after two years of continuous service, meaning younger employees who are newer to their jobs are more likely to miss out.
Young people are also much more likely to be on zero-hours contracts meaning they miss out on the right to request flexible working or the right to return to the same job after maternity, adoption, paternity or shared parental leave.
TUC general secretary Paul Nowak said: “Every worker should be protected from being sacked for no reason, but three in four young workers can be fired at will by bad bosses. Just imagine working hard in a job for nearly two years, only to be let go with no recourse.
“Too many young workers are trapped in insecure work, on lower pay and without the workplace rights most of us take for granted. That’s not right.”
Read more: Work jargon isolates younger employees
The study found 13% of 16-to-24-year-olds in employment are employed on a zero-hours contract, meaning they are more than five times more likely to be on these contracts than those aged 25 and over.
Young women are even more likely to be on zero-hours contracts (16%), as are young black, minority and ethnic minority workers (16%).
Fiona McKee said HR must support young workers who struggle to speak up about their rights at work.
She told HR magazine: “When young workers join the labour market it is often a time of uncertainty for them as they are going into a new environment for the first time and are unsure what to expect.
“HR must also ensure that the younger workers themselves feel supported and understand their entitlement to additional breaks and know who to contact for advice.
"Young workers can benefit hugely from having a work buddy within the team so that they can have a point of contact and someone they can easily access with any questions they may have.”
Young workers also face a higher unemployment rate than older workers.
Read more: Under appreciated young workers on brink of exodus
The unemployment rate for under-25s (12%) is nearly three times as high as that for all workers (4.2%).
Almost one in eight young people (12%) are without a job despite actively seeking work and being available to start work.
Median hourly pay for 16-to-17-year-olds is £8 per hour and £10.90 for 18-to-21-year-olds, compared with £15.83 for all employees.
Currently, the national living wage (£10.42 per hour) is not mandatory until an employee is 23.
However, from April next year, this will rise to £11.44 and expand to 21- and 22-year-olds.
The rate for apprentices will also go up to £8.60 for 18-to-20-year-olds, and to £6.40 for 16-to-17-year-olds and apprentices.
