Clearwire Corp shareholders should vote in favor of a proposed acquisition by Sprint Nextel Corp, given a lack of better options to keep the company afloat, influential proxy advisory firm ISS said on Friday.

Sprint, which already owns 52 percent of Clearwire's shares, has proposed paying $2.97 per share for the rest. Investors are due to vote on that bid May 21; a majority of the non-Sprint shareholders have to approve the deal for it to pass.

Some large minority shareholders, most notably Crest Financial and Aurelius Capital, have vehemently opposed the Sprint bid, saying it undervalued Clearwire. Aurelius sued Clearwire's directors last month, and Crest launched a proxy contest this week to block the deal.

But ISS said the Sprint offer looked fairly valued and that Clearwire appeared "increasingly unviable on a stand-alone basis." The firm also said certain governance provisions give Sprint an effective veto over other options anyway.

ISS, which researches proxy issues and makes voting recommendations to insti