Trade unions have hit out at a "ministerial plot" to scrap the Working Time Directive.
Brexiteers have gained widespread Cabinet support to axe the directive which limits the working week to 48 hours, the Sun on Sunday has reported.
Brexit-backing ministers are set to be demand an end to the EU rule when Cabinet meets on Monday (December 18) to decide what to demand when trade talks begin next year.
TUC General Secretary Frances O'Grady said the move was a "straight-up attack" on worker's rights.
She said: "Millions could lose their paid holidays, and be forced to work ridiculously long hours.
"The Working Time Directive gave nearly five million women paid holidays for the first time.
"No-one voted for Brexit to lose out on holidays, or to hand power over to bad bosses.
"The Prime Minister promised that our working rights would be protected after Brexit. Now we will see if she can keep her word, or if she is a hostage to extremists in her own cabinet."
The TUC have said that if the directive is scrapped seven million workers could lose rights to paid holidays - 4.7 million of them women - and many on zero-hours or part-time contracts.
Workers could also lose the right to lunch and rest breaks, and night workers could lose some health and safety protections, according to the TUC.