TMZ has learned ... a bodyguard who claims to have been sexually harassed by Britney Spears is filing a lawsuit today, claiming Britney repeatedly exposed herself to him and abused her children.

According to the suit, a copy of which TMZ has obtained, Fernando Flores claims Britney made "repeated unwanted sexual advances" toward him by exposing herself in front of him in her home.

Flores alleges, on one occasion -- in a scene right out of "Legally Blonde" -- "She was wearing a white lace, see-through dress. She walked over close by [Flores], intentionally dropped her cigarette lighter on the floor, bent over to retrieve it and thereby exposed her uncovered genitals to [Flores]." The lawsuit goes on, "The incident caused [Flores] shock and disgust."

The suit then goes on to describe another alleged instance where Britney summoned Flores to her room, where she was standing naked. The suit claims, "After an awkward silence during which [Britney] just stood naked before [Flores], [Flores] asked Spears if she needed anything. After some hesitation, Spears asked [Flores] to get her two bottles of 7 Up."

One time after allegedly exposing herself, the suit claims Britney said, "You know you liked it."

And the suit says, "In addition to exposing herself to [Flores], Spears engaged in numerous sex acts in front of [Flores]." There's also an allegation Britney was "loudly having sexual relations" in a hotel suite where her kids were present. The suit does not say if the kids saw anything.

And there is a more serious allegation -- The suit claims on one occasion Britney allegedly demanded Flores' belt, then walked in the house and allegedly twice "savagely hit the small child [Preston]" with the belt.

And the suit claims on another occasion Britney fed both kids crabmeat even though she knew they had serious fish allergies. The suit claims both kids started vomiting and Spears allegedly prevented anyone from seeking medical help, allegedly telling them, "Mind your own f**king business!"

Although it is not mentioned in the lawsuit, sources have told TMZ ... Flores contacted the L.A. County Department of Children and Family Services. And we're told DCFS investigated and determined the claim had no merit.

The suit will be filed this afternoon in L.A. County Superior Court.