Northrop Grumman Corp. defended the expert witness it tapped to testify in support of its retirement plan fees, saying “no basis in law” existed to disqualify her from the case.

The company is responding to a motion to disqualify attorney Marcia Wagner from testifying on the grounds that one of Wagner’s legal partners previously worked for the law firm representing the Northrop plan participants. The participants say a partner at Wagner’s firm, Thomas E. Clark Jr., previously worked for...