At first sight, the house of worship bears all the Persian architectural hallmarks of a classically Iranian mosque. But the religious symbol accompanying the sky-blue dome is a cross, not a minaret, and the holy book sacred to the faithful inside is the Bible rather than the Qur'an.

This is the Church of St Simon the Zealot, an Anglican church built in the late 1930s by an English missionary, Rev Norman Sharp, who went to Iran to spread the Christian faith.

Hidden behind a high wall in a narrow back street, it is now a haven to the tiny beleaguered Christian community in Shiraz, about 500 miles south of Iran's capital, Tehran.

Every Sunday, its 30-member congregation gathers in lamb-shaped pews - designed to symbolise Jesus' flock - to sing traditional hymns such as The Lord Is My Shepherd and hear sermons from a Farsi translation of the New Testament by the church's lay pastor, Stephen Kambiz Jaeintan.

Two days ago, this hardened society of believers met for a special Christmas Day dinner, having prepared and brought their own food. To inject an extra dose of festive cheer otherwise lacking in Iran's staunchly Islamic setting, a fully-dressed Santa Claus handed out presents.

In staging this hearty celebration of the birth of Christ, the congregation was risking the wrath of Iran's Islamic authorities, whose intelligence services keep a watchful eye on the church on such occasions. The entire flock has converted to Christianity from Islam, apostasy in a country whose population is 99% Muslim. Under Islamic law, such conversions are potentially punishable by death.

"We have big problems with the government," said Mr Jaeintan, 33, a once-devout Muslim who converted 14 years ago. "The authorities monitor the church to see who goes into the services. The entry of non-Christians is strictly forbidden.

"We are suffering repression for worshipping a God and the problems are getting worse. I am not allowed to travel abroad to study to be ordained as a priest. The most important thing for the authorities is that Iran remains an Islamic republic, with the Islamic part being more important than the republic.

"I was called into an interrogation with the intelligence service. They told me that the period when people were killed for being Christian is past but that I might find myself with two kilos of heroin in my possession. The punishment for that is life in prison or death. They told me they won't make a hero out of me."

Iran's constitution grants protection to Armenian, Assyrian and Chaldean Christians, as well as to Jews and Zoroastrians. After the 1979 Islamic revolution, Iran's religious leader, Ayatollah Ruhollah Khomeini, issued a fatwa ordering that Christians and Jews be treated with respect as "people of the book".

However, the edict does not sanction the conversion of Muslims to another faith, which is seen as a threat to the Shia Islamic foundations of the state.

At the very least, such conversions can damage work and educational opportunities. Workers in Iran's large public sector are screened for adherence to Islam while university applicants have to state their religion before being admitted. As a result, many converts feel compelled to hide their changed religious convictions.

"I work for the government in a job where you are required to carry out Islamic prayers every day. It's torture for me," said Nathaniel, 42, whose former Islamic beliefs were shaken after reading the Sermon on the Mount in the Book of Matthew. "I feel increased pressure every day. There is an ever present threat, but I feel when God is with me, who is against me?"

Despite being legally prohibited from actively seeking new members or publicising his services, Mr Jaeintan says the number asking to join his flock is increasing. Many are formerly observant Muslims like Nathaniel who have begun to question Islam; others are secular Iranians claiming to have experienced a spiritual awakening. Some, whom Mr Jaeintan says he rejects, want to become Christians as a means of seeking political asylum.

The repression of Christians in Iran predates the seventh-century arrival of Islam. Christians were brutally purged by the ardently Zoroastrian Sassanian dynasty during the third and fourth centuries. The Shiraz church is named after Simon the Zealot, a Christian patriarch put to death in 339AD by order of the country's rulers.

The US State Department has published reports on religious freedom lamenting the closure of many Iranian churches and noting the murders of several evangelical Christians in the 1990s. There were once several Anglican churches throughout Iran, most of them designed by the Rev Sharp. Today, the Church of St Simon the Zealot is one of Iran's few remaining centres of active Anglican worship.

Weeks after the revolution, the church's then pastor, Parviz Sayaphsina Arastu, was beheaded in the churchyard by religious extremists who accused him of carrying out baptisms.

More recent baptisms have been conducted in secret by pastors visiting from Britain and elsewhere. Mr Jaeintan is unqualified to baptise converts because he is not an ordained priest.

His lay status has also denied the church other basic Christian services such as Easter mass. Holy communion has been staged just once in the past year, with the help of a visiting Anglican priest.

Mr Jaeintan, however, remains defiant in his religious beliefs. "I'm proud of being a Muslim-born Christian living in Iran," he said. "My God has given me birth here, so it means I have a mission in Iran."

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