Analysis by the Trades Union Congress (TUC) estimates 1.2 million people are not getting any paid leave at all, while a million were not getting their minimum paid leave entitlement.
This equates to one in 12 workers missing out on nearly £3bn worth of paid leave a year, according to the TUC.
The sectors in which workers are most likely to lose out are agriculture (14.9%), mining and quarrying (14.7%) and accommodation and food (13.9%).
The TUC says the main reasons workers are losing out is the setting of unrealistic workloads that do not allow time to take leave, employers deliberately denying holiday requests and companies not not keeping up to date with the law.
Working people are entitled to a statutory annual minimum of 28 days paid leave (pro rata and including public holidays).
Minimum holiday entitlements are a vital part of reducing overwork, according to the TUC, which warned that working excessive hours increases the risk of developing heart disease, stress, mental illness, strokes, and diabetes.
The organisation is pressing the Government for a crackdown on employers who deny staff their statutory holiday entitlement.
This would include giving HM Revenue and Customs (HMRC) the power to ensure that workers are fully compensated for missed holidays.
The government has recently consulted on enforcing holiday entitlements but has yet to announce any plans.
TUC General Secretary Frances O’Grady has warned millions are missing out on holiday putting them “at risk of burnout”.
She said: “Employers have no excuse for robbing staff of their well-earned leave. UK workers put in billions of hours of unpaid overtime as it is.
“The government must toughen up enforcement to stop bosses cheating staff out of their leave.”