Frontier Airlines’ bid for Spirit faces an advisory firm’s opposition.

The prospects for a proposed sale of Spirit Airlines to Frontier Airlines appear to be under threat as a prominent shareholder advisory firm reversed its guidance on the combination of the two budget carriers, recommending on Friday that Spirit’s investors reject the deal.

A bidding war had led Spirit to repeatedly delay a shareholder vote as it sought to garner support for the Frontier deal over an unsolicited bid by JetBlue Airways. The additional time, shifting terms of the deals and market dynamics led the advisory firm, Institutional Shareholder Services, to change its recommendation twice, most recently on Friday.

That update comes just days after Frontier said that it had no plans to adjust its offer and acknowledged that it remained “very far” from obtaining the support needed from Spirit’s shareholders. At Frontier’s request, Spirit this week delayed the shareholder vote again, to July 27. It had been scheduled for Friday.

Although JetBlue’s offer is more valuable than Frontier’s, Spirit and Frontier have argued that regulators are unlikely to approve a sale to JetBlue. But after taking into account the potential effect of a regulatory delay, ISS estimated that JetBlue’s offer was still worth 11.5 percent to 19 percent more than Frontier’s.

Spirit’s management team has also repeatedly argued that the Frontier deal offers more long-term benefits to shareholders: The two airlines share a low-cost business model, and their combination would be easier to pull off and would create a viable national budget carrier, it has argued.

But movement in Frontier’s share price since it and Spirit jointly announced the acquisition plan in February suggests that investors are not confident that a market “recovery is on the horizon,” ISS said. Ultimately, it concluded, “the proposal from JetBlue appears to represent a superior alternative.”

Industry analysts generally agree that Spirit and Frontier are more complementary than Spirit and JetBlue, but combining two airlines is no easy task under any circumstance. The Biden administration is also expected to closely review either deal as it has broadly taken a skeptical view of large corporate mergers.