The survey of more than 1,000 managers also revealed that 35% of those who didn't take their full holiday entitlement lost at least one day overall by not being able to carry it across to the next year. One in five (20%) lost an entire week or more of holiday.
More than half (54%) of respondents said they feel the need to work when on holiday and 70% admit to reading and responding to work emails when on annual leave.
Almost half (40%) of managers reported there is room for improvement in their work-life balance, with 15% describing it as "poor".
ILM CEO Charles Elvin said the number of people losing a week or more of holiday is concerning.
"We know that work-life balance has a direct impact on performance," he said. "It’s important that workers feel they have time to switch off and a holiday is a big part of that.
"Managers have an important role to play in ensuring workers take the leave they are entitled to. I’m urging them to look at what hasn’t been taken and encourage their teams to take it.”