One of the nation’s largest public charities steered financial aid to patients of its two biggest corporate donors — the dialysis chains DaVita and Fresenius — while denying help to people who used smaller, unrelated clinics, in violation of anti-kickback laws, according to a federal whistle-blower lawsuit unsealed this week in Boston.

The charity, the American Kidney Fund, helps patients who need dialysis by paying their health insurance premiums and other costs for treatment. But under a longstanding federal agreement intended to prevent illegal kickbacks, the charity is supposed to provide help based solely on a patient’s financial need, and not favor companies that donate to it.

The lawsuit, filed by David Gonzalez, who worked for 12 years at the kidney fund in its patient assistance program until he left in 2015, accused the charity of creating a so-called blocked list of dialysis clinics whose patients would not get financial assistance while making sure patients at clinics operated by DaVita and Fresenius would.

The Justice Department, which had earlier issued subpoenas regarding the accusations, said it would not join the case. But it requested that the court seek the department’s consent if a decision were reached to dismiss or settle the lawsuit. Some cases that did not involve federal prosecutors have still led to large settlements, including one in which Celgene agreed to pay $280 million to settle claims it had inappropriately marketed the cancer drugs Thalomid and Revlimid.