Proxy advisory firm Institutional Shareholder Services is recommending Cigna Corp. shareholders vote in the favor of the health insurer's proposed acquisition of Express Scripts Holding Co.

ISS' recommendation stands in contrast to opposition from activist investor Carl Icahn, who is urging Cigna shareholders to vote against the deal.

Describing the proposed merger as a "financially compelling transaction," ISS said the transaction will give the combined entity immediate scale with strong cash flow generation. The deal should be should be significantly accretive to Cigna's EPS, the proxy advisory firm said.

ISS' views echoed that of Cigna shareholder Glenview Capital Management, which has backed the deal, saying that the combined company will save customers "billions of incremental dollars annually" and lead to "better healthcare outcomes."

Cigna shareholders are scheduled to vote on the merger Aug. 24.