The oldest-known Greek copy of an apocryphal Christian writing about the secret teachings of Jesus' to his brother James has been uncovered at Oxford University.

Geoffrey Smith and Brent Landau, scholars at The University of Texas at Austin discovered fifth or sixth century fragments of the ancient, controversial text known as the First Apocalypse of James, of which only Coptic translations were thought to have survived.

'To say that we were excited once we realised what we'd found is an understatement,' said Smith, an assistant professor in religious studies, who found the small fragment earlier this year amongst Oxford University archives. 'We never suspected that Greek fragments of the First Apocalypse of James survived from antiquity. But there they were, right in front of us.'

The Apocalypse is one of 13 Gnostic texts previously found written in Coptic in Egypt during 1945, collectively known as the Nag Hammadi library.

'The text supplements the biblical account of Jesus' life and ministry by allowing us access to conversations that purportedly took place between Jesus and his brother, James — secret teachings that allowed James to be a good teacher after Jesus' death,' Smith added.

The text is outside the traditional 27-book New Testament canon as set by Church father Athanasius, who decreed that the established texts could not be added to nor subtracted from in 367 AD. Gnosticism, a broad tradition which emphasised 'secret knowledge' revealed to a select few, was a heresy frequently contested with by the early Church.

Landau suggested the text has been used in an 'educational context', and produced by someone who 'had a particular affinity' for the outlawed manuscript. The historic discovery was announced at the prestigious Annual Meeting of the Society of Biblical Literature, hosted last month in Boston.