Around a million workers are at high risk of being forced to work excessive hours if the UK leaves the European Union, trade unions have claimed.
The TUC published analysis showing that since the introduction of the EU's Working Time Directive in 1998, the number of UK employees working an average of more than 48 hours a week has fallen by around half a million from 3,992,000 to 3,494,000.
But the size of the workforce has increased by 13.4% over that period, so the TUC estimates that without the directive, the ranks of those working excessively long hours would now top 4.5 million.
The Working Time Directive stipulates an average limit of 48 hours on maximum weekly working time, usually calculated over 17 weeks, with opt-outs available for individual workers.
TUC general secretary Frances O'Grady said that it had benefited employees' family life, health and safety, as well as protecting public safety by reducing the risk of tired workers in areas like health and transport making potentially fatal mistakes.
"Working people's rights are on the line in this referendum - and working time protections are particularly at risk," said Ms O'Grady.
"Brexit campaigners have made no secret of their wish to scrap working time protections. If they get their way, the 48-hour limit will be gone and your boss will be able to force you to work 60 or 70-hour weeks.
"The only way working people can be sure of keeping their rights at work is to stay in the EU. Nobody knows exactly how bad things could get for workers' rights outside of the EU, but the legal experts are all saying it will be worse."
Professions most likely to involve long working hours include mining and quarrying, where 33.1% of employees put in more than 48 hours a week in 2015, agriculture, forestry and fishing (28.5%), transport and storage (21.3%), construction (20.2%) and education (17%).
Shadow business secretary Angela Eagle said: "When Brexit campaigners talk about cutting 'red tape' we know what they mean.
"Their vision is a race to the bottom, a race where rights at work are slashed, and where working people will inevitably lose.
"If we left the EU, working people would be at risk of being forced to work more hours, meaning less time to spend with the family and more stress at work.
"And many other rights would be at risk too; from paid holidays and parental leave, to equal pay and anti-discrimination laws.
"Our membership of the EU is good for business and good for working people too. That's why Labour will be campaigning hard to remain."