Carl Icahn has a stake in Cigna and may oppose its deal to acquire pharmacy benefit manager Express Scripts, sources tell CNBC's David Faber.
The sources said the shareholder bases of Cigna and Express Scripts have significant overlap. Cigna and Express Scripts shareholders are set to vote on the deal on Aug. 24.
Cigna announced the $54 billion deal in March amid a wave of consolidation in the health-care industry. Cigna's rival Aetna had agreed to combine with drugstore chain and pharmacy benefit manager CVS Health months earlier in a deal that shook up the space.
Cigna CEO David Cordani has defended the deal as a way to "drive change" in a market that's currently "not sustainable."
The Wall Street Journal first reported Icahn's stake in Cigna Wednesday.
Cigna's stock gained 1.3 percent Thursday, while Express Scripts shares rose 4.4 percent.
—CNBC's Angelica LaVito contributed to this report.