Carl Icahn is urging Cigna shareholders to reject the health insurer’s attempted multibillion-dollar takeover of Express Scripts, saying it’s paying too much for a company with a shaky future.

Shares of Express Scripts were slightly up in morning trading Tuesday.

The billionaire and activist investor warned that Express Scripts, a pharmacy benefits manager, could face substantial regulatory risks and intense competition from Amazon. Cigna said in March that it would pay $52 billion for Express Scripts, a deal on which shareholders will vote Aug. 24.

“Cigna is dramatically overpaying for a highly challenged Express Scripts that is facing existential risks on several fronts,” Icahn said in an open letter to shareholders.

The investor also warned that Express Scripts could lose business from other insurers, who might shy away from the company if a rival like Cigna starts running it.

Icahn is not a disinterested party.

He has built up a stake in Cigna, whose shares have risen almost 5 percent since the Wall Street Journal first reported Icahn’s opposition to the deal last week. He is also betting that shares of Express Scripts will fall, he acknowledged Tuesday. In the same period, shares of the St. Louis pharmacy benefits manager have fallen more than 3 percent.

Cigna did not immediately return calls from The Associated Press early Tuesday.