Yahoo has written an open letter to its shareholders urging them to support its current management board, ahead of a showdown vote scheduled for 1 August.

The letter urges shareholders not to vote for an alternatives slate of managers nominated by billionaire Carl Icahn, warning them that Icahn cannot deliver them the best returns.

"Icahn has made it clear that his only objective is to sell part or all of Yahoo to Microsoft. That fact, combined with his lack of an operating plan going forward, means that he will have no leverage to negotiate a fair deal with Microsoft. He has set himself up for failure," said Yahoo chairman Roy Bostock in the letter.

"We believe you cannot count on Microsoft to bail out Mr. Icahn’s misguided agenda, at least not on terms that are in the best interests of Yahoo stockholders," Bostock added.

Icahn wants shareholders to vote out Yahoo's current management team, which he believes blew takeover negotiations with Microsoft.

Many shareholders have expressed similar concerns about Yahoo's failure to conclude a deal with Microsoft, but it unclear whether they will be willing to vote for Icahn's slate.

Microsoft had recently said it would be prepared to reopen negotiations over a transaction for all of part of Yahoo, provided Icahn's management team were voted in.

But subsequently it made a renewed bid for Yahoo's search businesses, which was again rejected.

Microsoft launched its original bid for to acquire all of Yahoo for a deal at the time worth $44 billion. Microsoft then withdrew its offer, only to come back with an offer for Yahoo's search business. That too was rejected, as was the recently revised offer.

If Icahn is successful in his attempt to oust Yahoo's board, it is to be expected that some sort of deal could be reached – although whether Icahn can negotiate the best deal remains open to question.

If Icahn is unsuccessful, Yahoo's current board has confirmed they remain open to offers, and indeed would sell the company for $33 per share. But after months of fractious negotiations, concluding a deal with Microsoft will prove tough.