Al-Azhar's Grand Imam Ahmed El-Tayyeb said on Monday that a joint meeting between Sunni and Shia scholars is only possible if the latter issue clear religious edicts from major references in Iran and Iraq that prohibit insults of the prophet's companions and spouses.
He also called for an end to what he said were attempts to spread the Shia doctrine in Sunni states.
His comments came during a meeting with Iraqi Vice President Khodair Al-Khozaei, who said he is certain that Al-Azhar is capable of uniting the Muslim world.
The majority of the population in both Iraq and Iran are Shia Muslims.
In May 2013, El-Tayyeb chaired a meeting with Islamist forces – including scholars, Muslim Brotherhood members and Salafists – during which they declared their total rejection of "attempts to spread Shi'ism in Egypt".
A few weeks before the ouster of Islamist president Mohamed Morsi, four Egyptian Shias were beaten to death by a mob, a lynching blamed partly on sectarian passions whipped up by ultra-orthodox Salafist Muslim allies of the Islamist president.
There are no official records of the number of Shias in Egypt, which is predominantly Sunni. However, community members estimate them to be between 800,000 and just under 2 million.
Article 2012 of Egypt's constitution that was drafted by an Islamist-led panel during Morsi's rule stated that Sharia law, the main source of legislation, includes credible sources that are accepted "in Sunni doctrines and by the larger community."
The condition of being compatible with Sunni doctrine is no longer mentioned in the 2014 national charter, which states in its preamble that the reference for interpretation of Sharia law are the relevant texts in the collected rulings of the Supreme Constitutional Court.
Egypt's Al-Azhar, now the highest seat of Sunni religious authority, was originally a Shia place of worship when it was founded in 970 in order to spread Shia Islam throughout the world.
The Shia denomination emerged in the earliest days of Islam from a dispute over who should lead the Muslim community after the death of Prophet Mohamed. Shias believe leadership should have passed to Ali, the prophet's son-in-law, and his descendants, rather than Abu Bakr, Mohamed's companion.