Cigna says it is rejecting Anthem's proposed $48 billion acquisition bid and suing the Blue Cross-Blue Shield insurer.
The announcement comes hours after another major insurer, Aetna, said it was abandoning its planned $34 billion purchase of Medicare Advantage provider Humana.
The Department of Justice sued last summer to stop the deals due to concerns about how they would affect prices and consumer choices. Federal judges rejected both deals in separate rulings earlier this year.
Anthem Inc. had filed paperwork Monday to appeal its case.
Cigna Corp. says it is seeking a $1.85 billion termination fee from Anthem and more than $13 billion in damages caused by what it says were Anthem's breaches of the merger agreement.
An Anthem spokeswoman did not immediately return a call seeking comment.