Morocco's royal family is using the institutions of the state to "coerce and solicit bribes" in the country's lucrative real estate sector, according to a leaked report from American diplomats.

Information about high-level corruption involving the rulers of Washington's closest ally in north Africa was brought to the attention of the US consulate in Casablanca, Morocco's commercial capital, by a businessman in 2009, leading diplomats to describe "the appalling greed" of those close to King Mohammed VI.

According to the US report, decisions involving Omnium Nord Africain (ONA), a holding company owned by the king, are made only by the king and two of his powerful associates. "To have discussions with anyone else would be a waste of time," the head of the company is quoted as saying.

Royal involvement in business is a hot topic in Morocco but public discussion of it is sensitive. The US embassy in Rabat reported to Washington in a separate cable that "corruption is prevalent at all levels of Moroccan society".

Mohammed, who succeeded his father, Hassan, in 1999, is said to have cleaned up the royal family's act, but it appears he has not done enough.

"While corrupt practices existed during the reign of King Hassan II … they have become much more institutionalised with King Mohammed VI," one cable quotes a businessman as saying. Institutions such as ONA – Morocco's largest conglomerate, which clears most large development projects – regularly coerced developers into granting beneficial rights to ONA, the businessman was quoted as saying.

In 2008 ONA dismissed Saad Bendidi as chief executive for mismanaging Wana, the company's telecommunications subsidiary – a move that drew public attention to the king's business activities. "Despite the intrigue surrounding Bendidi's firing, members of the business community in Casablanca did not find it particularly unusual," the consulate reported. "A long-time franchise holder pointed out that Bendidi's predecessor … also did not last more than a few years at ONA's helm. As one businessperson put it, the palace can be very demanding. When the palace calls, 'if you don't pick up the phone on the first ring you're in trouble'."

Media comment included calls on the king "to exit the business world, citing the inherent conflict between his role as ultimate arbiter of the Moroccan system and leading businessman and banker within it".

The affair "cast doubt on the transparency of the king's business affairs", an "explosive situation" at a time when Moroccans face rising prices for goods whose production and distribution is often assured by the king's own companies. There has long been hushed debate of these issues in Moroccan business circles but "few expect the royal role in ONA to change anytime soon".

The experience of the US consulate's informant "demonstrates a reality, of which most Moroccans dare only whisper – the influence and commercial interest of the king and some of his advisers in virtually every major real estate project here", the cable records. "A former US ambassador to Morocco, who remains closely connected to the palace, separately lamented to us what he termed the appalling greed of those close to King Mohammed VI. This phenomenon seriously undermines the good governance that the Moroccan government is working hard to promote."