Almost two million workers are not receiving their legal holiday entitlement, partly because they do not have enough time to take all their leave, a new study suggests.
Research by the TUC indicated that one in 14 employees are missing out on the time they are due to take off.
Unrealistic workloads and employers denying holiday requests or not keeping up to date with the law were the main reasons, said the TUC.
READ MORE: These are the UK airports and airlines that are set to strike over summer
Women were more affected than men, while workers in education, health and social care and retail were most likely to be losing out, according to the report.
More workers are taking claims for unpaid holidays to an employment tribunal since fees were abolished in 2017, said the TUC.
General secretary Frances O'Grady said: "Every worker deserves a break to spend time off with friends and family, but millions are missing out on the paid leave they are owed.
"British workers put in billions of pounds worth of unpaid overtime as it is.
"Employers have no excuse for robbing staff of their leave.
"The Government must toughen up enforcement to stop bosses cheating working people out of their holidays and ministers must not resurrect tribunal fees which stopped people enforcing their rights."
Workers are entitled to 28 days holiday for a typical five-day week, although the TUC added that previous research revealed employees put in £32 billion worth of unpaid overtime last year.
The Government announced last week that millions of low-paid workers could receive more workplace protections under measures to advance its so-called "Good work plan", describing it as the largest upgrade to workers' rights in a generation.
Proposals included the creation of a single labour market enforcement body, which will have the powers to enforce minimum wage and holiday payments.