One in 20 UK workers don't receive paid holidays
Around one in 20 workers do not receive any paid holidays, and one in 10 does not get a payslip, according to a think-tank.
The Resolution Foundation said its study highlights the scale of unlawful working practices across the UK.
Its research indicates that workers over the age of 65 are most likely to not have paid holidays, despite the legal entitlement to 28 days a year, or pro-rota for part-timers.
The older age group are also more likely than younger workers to earn less than the minimum wage, said the report.
The Resolution Foundation said workers in hotels and restaurants miss out out more than others on legal workplace entitlements, while those in small firms, employing fewer than 25, are most likely to not get pay slips and paid leave, as are workers on zero-hours and temporary contracts.
The think-tank said the scale of working practice abuse highlights the need for better enforcement of the UK's labour market rules.
Lindsay Judge, senior economic analyst at the Resolution Foundation, said: "The UK has a multitude of rules to govern its labour market, from maximum hours to minimum pay, but these rules can only become a reality if they are properly enforced.
"Labour market violations remain far too common, with millions of workers missing out on basic entitlements to a pay slip, holiday entitlement and the minimum wage.
"The Government's welcome proposal to create a new single enforcement agency should leave it better placed to tackle these labour market violations than the multiple bodies currently operating, as long as it's properly empowered and resourced.
"Our analysis suggests that, while violations take place across the labour market, the Government should also prioritise investigations into sectors like hotels and restaurants, along with firms who make large use of atypical employment contracts, as that's where abuse is most prevalent."