Lawyers representing Mattel Inc. in its lawsuit against MGA Entertainment are claiming that Bratz doll designer Carter Bryant erased incriminating data from his personal computers that would have showed he designed the dolls while working for Mattel.

Bryant used a program called “Evidence Eliminator” on his computers in July 2004, two days before he was to turn them over to Mattel, lawyers for the El Segundo-based toymaker said Tuesday afternoon in federal court in Riverside.

If Mattel can prove Bryant designed the popular fashion dolls in 2000 while he worked for Mattel then secretly took the idea to MGA, it will likely reap hundreds of millions of dollars in lost revenues. Industry experts estimate that the Bratz line sold more than $500 million last year for Van Nuys-based MGA and has significantly eroded sales of Mattel’s flagship Barbie dolls. MGA is privately held.

Attorneys representing MGA acknowledged Bryant used the Evidence Eliminator program but said Bryant erased explicit Internet pop-up ads. Bryant and MGA have claimed that the designer came up with the idea for the dolls in 1998, during a period he wasn’t working for Mattel and was living with his parents in Missouri.

Mattel and Bryant settled a week before the trial started last month. Terms were not disclosed. Bryant is expected to testify as early as tomorrow.

Shares in Mattel were down 2.2 percent to $19.84 in early trading Wednesday.