He is one of America's most renowned rabbinic exports and a hero to many modern-Orthodox Jews around the world. But Israel's Chief Rabbinate does not share the admiration for Shlomo Riskin, and has now taken its gloves off in its fight against him.

Rabbi Riskin makes unacceptable changes to Jewish law and "causes conflict", Sephardi Chief Rabbi Yitzhak Yosef declared in a lecture last weekend.

He avoided referring to Rabbi Riskin by name, but called him the "person from Efrat" - the settlement where Riskin is Chief Rabbi - and left no room for doubt by assailing him on one of his rabbinic rulings.

This ruling, while seemingly obscure to many, cuts to the heart of why Israel's Chief Rabbinate is at loggerheads with Rabbi Riskin - it is all about the values that guide him. These are the very values that many modern-Orthodox Jews see as the essence of the "modern" aspect of their identity.

Rabbi Riskin has ruled that men alter the daily blessing that thanks God "who did not make me a woman", in order to shore up gender equality. It follows a pattern of earlier rulings, such as permitting women to dance with Torah scrolls in synagogue during Simchat Torah celebrations.

Rabbi Yosef told his predominantly Charedi audience: "We say the blessing every morning, that you didn't make me a woman. Not like some person from Efrat, who makes all kinds of changes, makes changes and causes conflict, who says you don't need to say the blessing - [who views] men and women with equal rights."

The fight began last month. Rabbi Riskin's supporters were appalled when they heard that he is likely to be retired as Chief Rabbi of Efrat, a settlement he helped to found upon emigrating from the US in 1983. Instead of granting an automatic approval to serve beyond the default retirement age - 75, which he reached last month - the decision-making body in the Chief Rabbinate is leaning towards retiring him, reportedly because of his liberal stances.

He is, in the Orthodox context, radical. He is taking the monopoly on halachic decision-making out of the hands of men by training and qualifying female religious leaders. In January, he appointed one of the graduates, Jennie Rosenfeld, as a spiritual adviser for Efrat's residents.

As well as challenging established practices such as the blessing, and changing the definition of who controls Jewish law, Rabbi Riskin is also causing controversy on the issue of who can enter Jewry.

His rabbinic superiors, he believes, are making conversion far too difficult and he bitterly complains that they must ease the process. There are even reliable rumours that he has ignored the Chief Rabbinate's national monopoly on conversion and started performing his own conversions.

The Riskin affair has laid bare the depth of the conflict within Israel's rabbinate between the Charedi rabbis, who are in control, and the modern-Orthodox rabbis, who want institutional change and are becoming more frustrated when it does not happen. If Rabbi Riskin is retired, it could spark something of a rabbinate civil war.