Half of workers on zero hours contracts and two-fifths of those on temporary contracts wrongly believe they are not entitled to paid holidays, a report shows.
A study by Citizens Advice found some employers were misleading workers about holiday entitlement due to bosses' ignorance, but others were deliberately flouting the law and exploiting workers' confusion.
The charity called on the next government to ensure workers are aware of, and are able to take, the paid holiday that they are entitled to.
In the last financial year almost 185,000 people got help from Citizens Advice on employment issues, with 10,000 cases specifically about paid holiday.
Chief executive Gillian Guy said: 'Thousands are missing out on rights they are entitled to due to a lack of awareness, confusion and in some cases deliberate dirty tactics by employers.
'With more than half of employers having staff working shifts or variable hours, action needs to be taken now to protect workers' rights.'
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