No one in Egypt can agree on how many people live in Cairo, let alone the precise ratio of Muslims to Christians. But senior government clerics are quite sure of one thing: there are exactly 866 atheists in Egypt – roughly 0.001% of the population.

This suspiciously precise figure means Egypt harbours the highest number of atheists in the Arab world, according to claims by Dar al-Ifta, an official wing of government that issues religious edicts, citing research released this week by a regional polling group. Morocco came in second, with supposedly only 325 atheists. Yemen is meant to have 32.

Religiosity is very high in Egypt, and across the Arab world. But the tiny estimates nevertheless prompted high amusement among atheists and secularists in Egypt, who say atheism is slowly on the rise. Even Dar al-Ifta’s definitions of atheism seemed comic. According to the clerics, atheists include not just unbelievers, but those who believe in a secular state, and Muslims who convert to other religions.

“They are in denial,” said Rabab Kamal, a spokesperson for The Secularists, a small but vocal group that lobbies for a secular state. “I could count more than that number of atheists at al-Azhar university alone,” she added, referencing the Cairo-based institution that is widely regarded as the seat of global Sunni learning.

“In pragmatic terms, you can’t make scientific studies about how many atheists or agnostics there are – we’re in a country where talking about ideology other than Islam is a stigma.”

Dar al-Ifta clerics say the number of atheists in Egypt is a dangerous development that should “set alarm bells ringing” – a stance that may surprise outsiders who imagined last year’s overthrow of the Muslim Brotherhood constituted a stepping stone towards a secular state.

While the government has launched a crackdown on the Brotherhood and other more extreme Islamist groups, mosque and state are just as interdependent as ever in Egypt.

When the constitution was rewritten in January, there was no question of removing the decades-old article 2, which stresses that Islamic teachings should form the basis for legislation. The country’s current leadership still base much of their legitimacy in the support bestowed by Islamic institutions – hence their aversion to atheism, which renders that support meaningless. Their primary problem with the Brotherhood was not that the group wanted a religious state, but that its vision of what that entailed undermined Egypt’s old hierarchy.

“The ruling regime in Egypt tends to be conservative, in order to maintain the cultural order,” said Ahmed Samer, the founder of The Secularists. “So any change would be worrying for them whether that was atheism or fundamentalism.”

Government officials may also be conscious that any slide towards secularism risks playing into the hands of the Brotherhood, who have portrayed their overthrow as a war on Islam itself, rather than on just one particular Islamic group. “The increase in the atheist wave could support the Islamists’ stance that only they are the guardians of religion,” explains Samer. “It will be an opportunity for the Islamists to gain a bigger base in the streets.”

Additional reporting by Manu Abdo

• This article was amended on 15 December 2014. An earlier version said the 866 atheists in Egypt represented roughly 0.00001% of the population. That has been corrected to roughly 0.001%.