In a stern letter that threatens legal action against the young star, James G. Robinson, the CEO of Morgan Creek Productions, calls Lohan's recent absences from the set "irresponsible and unprofessional" and accuses her of acting "like a spoiled child."
The letter, dated July 26 and marked for hand delivery to the actress at the Chateau Marmont hotel, was first posted on website The Smoking Gun.
A representative for Morgan Creek has confirmed the authenticity of the letter.
Early on Wednesday morning, Lohan was briefly hospitalized for heatstroke and exhaustion.
The 20-year-old was spotted out the night before she became ill with pal Nicole Richie singing karaoke at West Hollywood nightclub Guy's.
Robinson's letter adds, "We are well aware that your ongoing all-night heavy partying is the real reason for your so called 'exhaustion'."
A source close to the actress tells People, "She hasn't been crazy partying. In fact, I've never seen her so together."
A publicist for Morgan Creek Productions said on Thursday, "Lindsay reported to the set today on time. We're hoping she remains on schedule for the rest of the production."
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