A leading proxy advisory firm has urged Nissan Motor Co. shareholders to vote against the reappointment of its chief executive as a director, heaping more pressure on Hiroto Saikawa as he struggles to find accord with alliance partner Renault SA.

The move marks a rare public rebuke by an international proxy firm against the chief executive of a top-tier Japanese firm. It underscores the precarious position of Saikawa, who was groomed for leadership by ousted Chairman Carlos Ghosn but appears unable to mend the wide rift with Renault — a relationship one source said appeared to be in jeopardy.

Institutional Shareholder Services Inc. recommended that shareholders vote against Saikawa as director at Nissan's annual general meeting this month, to ensure a "clean break" from the Ghosn era. The former chairman, first arrested in November, is awaiting trial on financial misconduct charges. He denies all the charges against him.