The relationship, which stretched back almost 40 years, went beyond simple exchanges of intelligence on common Middle Eastern enemies. Israel reportedly trained Moroccan agents, supplied the king with tanks, and played a key - if subordinate - role in the assassination of an opposition leader.
Secret contacts began in 1961 under King Hassan's father, Mohammed V, who allowed tens of thousands of Moroccan Jews to emigrate to Israel. His son broadened and institutionalised the links after Meir Amit, the then head of Mossad, Israel's external security service, stole into Marrakesh to meet him in 1964.
Local and foreign reports this week revealed just how deep the mutually beneficial relationship went. Much of the detail has never been published before. Mossad provided technical assistance and training for its Moroccan sister organisation, as well as information on dissidents plotting to assassinate the young king.
Yossi Melman, co-author of a study of Israeli intelligence, disclosed in the liberal daily Ha'aretz that Mossad also relayed material on the subversive intentions of Egypt's revolutionary leader, Gamal Abdel Nasser.
According to Oded Granot, a writer on Arab affairs in the tabloid Ma'ariv, Mossad delivered more than 100 light tanks to Morocco in the Sixties to strengthen King Hassan in his conflict with Algeria over the Spanish Sahara.
Relations cooled in 1965, when Mossad's hand was revealed in the murder of one of King Hassan's political foes, Mahdi Ben-Barqa. Israel, according to another Ha'aretz writer, Amir Oren, helped track him, but did not kill him. The two services distanced themselves from each other, but cooperation soon resumed.
Mr Oren said the Moroccan secret service enabled Mossad to eavesdrop on conversations between national leaders and army officers at an Arab summit called to discuss a united military command. "This surveillance," he wrote, "was critical to the intelligence mosaic that led to Israel's victory in the 1967 Six-Day War."
Relations soured again after the 1973 Yom Kippur War, when Morocco infuriated Israelis by sending a token brigade to Syria as a gesture of solidarity. But three years later, Yitzhak Rabin became the first Israeli prime minister to pay a clandestine visit to Morocco. Mr Rabin, disguised in a Beatles- style wig and glasses, explored with the king the prospects for peace with Egypt and Jordan.
After Menachem Begin's right-wing Likud party came to power in 1977, King Hassan hosted discreet talks between the then Israeli foreign minister, Moshe Dayan, and Anwar Sadat's special envoy, Hassan Tohami. Mr Dayan slipped in from France, sporting a wig, moustache and dark glasses instead of his eyepatch. The dialogue persuaded Mr Sadat to fly to Jerusalem, paving the way for the 1979 peace treaty.
Although Israel and Morocco have still not established full diplomatic relations, the king openly hosted visits by two Labour prime ministers - Mr Rabin and Shimon Peres - in the Nineties.