Billionaire Investor Carl Icahn urged Cigna Corp. (CI - Get Report) shareholders Tuesday to reject the company's planned $60 billion acquisition of Express Scripts Holdings Co., (ESRX) calling the deal a "folly" and warning that Amazon Inc. (AMZN - Get Report) is posing an "existential threat" to the planned business model.

Icahn reportedly owns a less than 5% stake in Cigna and believes it's paying too much for Express Scripts, the country's largest pharmacy-benefit management group. Shareholders of both companies are scheduled to cast ballots on the combination on Aug. 24.

"Competitive risk from Amazon, arguably the strongest competitor in the world, will be an existential threat to PBMs like Express Scripts, possibly challenging their very existence," the letter states. "As an alternative, we believe Cigna should pursue a multi-year partnership with an existing PBM provider, potentially Express Scripts, while the industry resolves ... structural challenges."

"During this time, management can further develop or acquire their own PBM capabilities optimized for the rapidly changing regulatory and competitive environment," Icahn argued.

Cigna shares were marked 0.012% higher in pre-market trading Monday, indicating an opening bell price of $187.88 each, a move that would put its year-to-date decline to around 7.5% Express Scripts was marked 2.06% lower at $75.25 each.

Only shareholders who have held a stake on July 10, the record date, or before are entitled to vote at a special meeting Aug. 24 on the Express Scripts merger. According to a person familiar with the situation, Icahn had been accumulating shares in Cigna since before July 10 and plans to vote those shares to block the deal.

The position may put him at odds with sometimes activist Larry Robbins and his Glenview Capital Management LLC, who said April 23, he supports Cigna's acquisition. Express Scripts, the country's largest benefits management group, unveiled the $67 billion plan in March.