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

“Purchasing Express Scripts may well become one of the worst blunders in corporate history, ranking up there with the Time Warner/AOL fiasco and General Electric’s long-running string of value destruction.”

Shares of Express Scripts dipped in premarket 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 to open a letter.

“Purchasing Express Scripts may well become one of the worst blunders in corporate history, ranking up there with the Time Warner/AOL fiasco and General Electric’s long-running string of value destruction,” Icahn said in the letter.

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.

Pharmacy benefits managers, or PBMs, run prescription drug coverage for big employer and insurers, among other clients.

They’ve become a focal point for criticism as drug prices continue to rise, particularly for the rebates pharmaceutical companies give PBMs in order to get drugs included in a formulary, or list of covered medicines. Critics say these rebates play a major role in keeping drug prices elevated, and regulators have talked about eliminating them.

Icahn noted that the government’s direct challenge “to the highly flawed rebate system” along with Amazon’s “almost certain” entrance into the market as a competitor make any arguments in favor of the deal appear weak.

The investor suggested that Cigna Corp., based in Bloomfield, Connecticut, instead consider multi-year partnerships with a pharmacy benefits manager, possibly even Express Scripts, while the healthcare sector deals with mounting.

Express Scripts and other PBMs have said they pass the vast majority of the rebates they receive on to customers like big employers, and those rebates help restrain price growth.

Amazon has not announced plans to create a PBM, but talk of the online giant’s potential entrance into this market has created a level of uncertainty in the sector.

Amazon has formed a new venture with JPMorgan Chase and Berkshire Hathaway to figure out fresh ways to attack rising health care costs for their U.S. employees and possibly for many more Americans. It also announced last month that it will buy the online pharmacy PillPack. Icahn noted that will compete with Express Scripts’ mail-order pharmacy business.

Cigna executives have said the Express Scripts deal will give the company better financial flexibility and improve its free cash flow, among other advantages. They also say the combination will improve service and give doctors a more complete picture of patients.

Shares of Express Scripts Holding Co. fell another 2 percent Tuesday before the opening bell. Shares of Cigna Corp. were unchanged early Tuesday.

The Associated Press contributed to this report.