The studio producing Lindsay Lohan's upcoming picture, "Georgia Rule," is furious with the club-hopping star, and a senior producer says it's her "all night partying" -- and not "heat exhaustion" -- that's causing her to miss work.
The company producing "Georgia Rule," which began filming earlier this month, has accused Lohan of causing "hundreds of thousands of dollars" in damages, and is threatening to sue the 20-year-old star.
"We refuse to accept bogus excuses for your behavior," James G. Robinson, the CEO of Morgan Creek Productions, said in a July 26 letter to Lohan, describing her as being "discourteous, irresponsible and unprofessional" and acting "like a spoiled child"
On Tuesday Lohan was rushed from the set of "Georgia Rule" to a hospital in Los Angeles after she "got overheated and dehydrated" from filming in 105 degree weather, according to her publicist, Leslie Sloane-Zelnick. She received a vitamin B-12 shot and was hydrated and released that same day.
But Lohan has been seen around town at parties all week. And on the night of her trip to the hospital, she was seen entering Guy's Bar, an L.A. hot spot, at 11 p.m. with a group of pals, including boyfriend Harry Morton, 25, and close pal 24-year-old Nicole Richie.
On Sunday she was at Malibu's Polaroid Beach House for Jeremy Piven's birthday party.
"Since the commencement of principal photography of 'Georgia Rule,' you have frequently failed to arrive on time to the set," Robinson writes in the letter.
"Today [July 26], you did not show for work (all day). I am now told you don't plan to come to work tomorrow because you are not feeling well. You and your representatives have told us that your various late arrivals and absences from the set have been the result of illness; today we were told it was 'heat exhaustion.'
"We are well aware that your ongoing all night ... partying is the real reason for your so called 'exhaustion.'"
The letter was posted Friday on the Web site Thesmokinggun.com. Greg Mielcarz, a Morgan Creek spokesman, confirmed to ABCNews.com that the letter is authentic but would offer no further comment.
In "Georgia Rule," Lohan plays a rebellious teen who is sent by her dysfunctional mother (played by Felicity Huffman) to spend a summer with her grandmother (played by Jane Fonda).
ABC News Radio's David Blaustein contributed to this report.