Shareholder Advisory Firm Backs Spirit-Frontier Merger

The influential shareholder advisory service Institutional Shareholder Services is now advising Spirit Airlines’ investors to vote in favor of a proposed merger with Frontier Airlines over a rival offer from JetBlue Airways, after Frontier increased its offer.

The recommendation, which was delivered on Friday, is a reversal for ISS, which had previously advised Spirit shareholders to vote against a deal with Frontier, a fellow low-cost…