The odds are mounting against Michael Dell’s $24.4 billion bid to take Dell Inc. private.

Institutional Shareholder Services, an investor-advisory firm, is leaning against the deal, according to people with knowledge of the matter. A negative recommendation by the influential firm would increase the likelihood that investors will oppose the proposal -- unless Michael Dell and his partners sweeten the $13.65-a-share bid, said the people, who asked not to be identified because the talks are private.

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