As if Megiddo, the biblical city of Armageddon - scene of three millennia of battles, the last cavalry charge of the first world war and the final showdown between good and evil - did not have enough on its plate. Archaeologists now claim to have unearthed the remains of the oldest Christian church discovered in the Holy Land.

Unfortunately for Israel's beleaguered tourism industry, the find was made behind the walls of one of the country's maximum security prisons.

Inmates were put to work alongside the specialists to excavate a corner of Megiddo jail for the construction of a new cell block ready for the next intake of Hamas and Islamic Jihad militants.

Toiling behind the barbed wire and watchtowers, they uncovered a detailed and well-preserved mosaic, the foundations of a rectangular building, and pottery dated to the third or early fourth century. One of several inscriptions on the mosaic floor in ancient Greek said the building was dedicated to "the memory of the Lord Jesus Christ".

Other inscriptions name a Roman army officer, Gaianus, who donated money to build the floor, and a woman called Ekoptos who "donated this table to the God Jesus Christ in commemoration". The table is believed to have served as an altar.

"There are no crosses on the mosaic floor," said Yotam Tepper, an archaeologist who led the dig on behalf of the Israeli Antiquities Authority. "In their place is a picture of two fish lying side by side - a very early Christian symbol.

"This is an extremely dramatic discovery, because such an old building of this type has never been found either in the land of Israel or anywhere else in the entire region. The structure and the mosaic floor date back to the period before Christianity became an officially recognised religion, before St Constantine.

"Normally we have from this period in our region historical evidence from literature, not archaeological evidence. There is no structure you can compare it to - it is a unique find."

The Roman empire forbade Christian rituals before AD313 and Christians were forced to worship in secret. The earliest churches, until this discovery at Megiddo, include the Holy Sepulchre in Jerusalem, said to stand on the site of Christ's crucifixion, dating from about AD330, and the church of the Nativity in Bethlehem. The inscriptions at Megiddo were interpreted by Professor Leah Di Segni of the Hebrew University.

"I was told these were Byzantine but they seem much earlier than anything I have seen so far from the Byzantine period. It could be from the third or the beginning of the fourth century," she said.The use of the word "table" in one inscription instead of "altar" might advance the study of Christianity, she said, because it is widely believed that rituals based on the Last Supper were held around a table used as an altar.

The church might never have been discovered had it not been for the needs of Israel's ever-demanding security apparatus. Megiddo prison is home to about 1,200 "security prisoners" who are held in "administrative detention" without ever being told exactly what it is they are accused of.

The prison is a series of fenced-in compounds with the bulk of inmates sleeping in long brown army tents enclosed by barbed wire and surrounded by open sewers. The prisoners nicknamed the jail "Jabaliya" after a poor and overcrowded refugee camp in the Gaza strip.

Megiddo has long been described by religious scholars and archaeologists as the most important biblical site in Israel. Over the centuries, more than 25 cities rose and fell at Megiddo. Some were powerful commercial centres on the ancient thoroughfare between Egypt and Mesopotamia.

Five of the conflicts fought in the 30-mile-wide Jezreel valley around Megiddo are recorded in the Old Testament. The New Testament names Armageddon - a Greek corruption of the Hebrew word "har", meaning mount, and Megiddo - as the scene of the final great battle between good and evil.

Some specialists remain sceptical about the latest discovery. "I think this is a little myth to boost tourism," said Michel Piccirillo, a respected biblical archaeologist. "The idea that it is ancient comes from the pottery and the shape of the letters on the inscriptions, but this is not definitive."

Israel's tourism minister, Avraham Hirchson, is not deterred. "If we nurture this properly, then there will be a large stream of tourists who could come to Israel. There is great potential ... " he told national television.