The indictment alleged that el-Sukkari stabbed Tamim to death after he tricked her into opening the door of her apartment by posing as a representative of the building's owners.
It accused Moustafa of taking part in the killing through "incitement, agreement and assistance".
Moustafa ordered el-Sukkari to kill Tamim after she married Riyad al-Azzawi, an Iraqi kick-boxing champion, the court heard.
Tamim had had a relationship with Moustafa over a three-year period that ended several months before her death, Egyptian media had reported.
Amr el-Kakhy, Al Jazeera's correspondent in Cairo, said that the verdict had come after unprecedented public interest in the trial.
"I think this is the biggest case in Egypt in this new millennium, [in terms of] the level of media coverage and public interest into the case, which led the court to put a gag order on publishing what went on throughout the court hearing," he reported.
The evidence against Moustafa and el-Sukkari included recordings of telephone calls between the two men, as well as footage from security cameras at the Dubai apartment building.
|El-Sukkari killed Tamim after tricking his way into her home, the court heard [EPA]
A DNA-link was found on the blood-stained clothes that el-Sukkari was accused of leaving near the scene of the crime.
Tamim, who was 30 when she was killed, won regional fame after winning a television talent contest in 1996.
Shares in Talaat Moustafa, which is now headed by Moustafa's brother, nosedived after the construction magnate was arrested in September.
They closed on Wednesday at 5.03 Egyptian pounds ($0.89) per share, off a year-low of 2.52 pounds in March.
Investors in the Talaat Moustafa group might now begin selling off their shares, el-Kakhy said.
"Analysts told me before the verdict that if Moustafa was found guilty, they were expecting a big sell-off of shares in the company," he said.