Yahoo Inc. is seeking to conceal large portions of a shareholder lawsuit alleging the Internet company’s board improperly thwarted Micrsoft Corp.’s $47.5 billion takeover offer, raising shareholder questions over the motives for the secrecy.

In a letter sent Friday to the judge overseeing the case in Delaware, a lawyer for the shareholders argued Yahoo is trying “to whitewash embarrassing documents” because the company thinks the information will damage the board’s efforts to repel a challenge by activist investor Carl Icahn.