A studio executive scolded Lindsay Lohan in a letter for her behavior on a movie set and doubted her reasons earlier this week when she said she suffered from heat exhaustion.

James G. Robinson, CEO of Morgan Creek Productions, sent the letter dated July 26 to the 20-year-old actress, as well as agents, producers and others involved in the film.

On Tuesday, Lohan's publicist told the syndicated entertainment show "The Insider" that her client was taken to a hospital, where she was treated for overheating and dehydration. Lohan had been filming "Georgia Rule" in 105-degree weather for 12 hours that day, spokeswoman Leslie Sloan Zelnick said.

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Robinson, 70, wasn't buying the excuse.

"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 heavy partying is the real reason for your so-called 'exhaustion,"' Robinson wrote in the letter posted Friday on TheSmokingGun.com.

The letter was authenticated by a spokesman for Morgan Creek, according to the Web site and the Los Angeles Times.

Phone messages left for Zelnick and Morgan Creek were not immediately returned late Friday.

Lohan plays a troubled teen in "Georgia Rule," which also stars Jane Fonda and Felicity Huffman. Robinson said in the letter that Lohan has been frequently late for filming and has been "discourteous, irresponsible and unprofessional."

"You have acted like a spoiled child and in so doing have alienated many of your co-workers and endangered the quality of this picture," Robinson wrote.

The studio executive also said if Lohan does not honor her commitments, the studio will pursue "full monetary damages" and take other necessary action to preserve the studio's financial interests.

The star of "Freaky Friday" and "Herbie: Fully Loaded," Lohan was treated at hospitals twice in January — in London for a cut on her leg and in the Miami area for an asthma attack.

In 2005, she was hospitalized for exhaustion and lost a noticeable amount of weight.