CHRISTIANITY in Portsmouth may be 500 years older than was first thought.

A discovery at a city church may persuade historians to re-write the story of how faith spread across the south coast.

Preliminary archaeological investigations suggest there may be a Saxon church dating from the seventh century under St Mary's Church in Fratton.

Experts already knew there was a Norman church from the 12th century under the existing building - the earliest known church site in the city. This latest find suggests the site could have been the cradle for Christianity in Portsmouth 500 years earlier than that.

Church leaders now hope the find will help them develop the site for use in the future. An ambitious four-year plan to provide better facilities for the community has now been unveiled, including re-developing the tower, organ and worship area.

The church will be holding an open day between 10am and 2pm on September 13 to show people the archaeological findings, give them a chance to dig for themselves in a mock-archaeological display, and explain the re-development work. Visitors can also climb the tower to enjoy views of the city, and examine old marriage and other registers.

Archaeologist Kate Longcliffe, a member of the St Mary's congregation, dug a trial trench at the north side of the church and then used sophisticated radar equipment to find out what was under the existing Victorian building.

Studies of the archives have shown previous churches on the site. Each of these buildings has been confirmed by the ground radar equipment including a mid-Victorian one 50cm below the existing floor and a post-Norman church 90cm below the present floor level. To everyone's surprise the radar found another structure at a depth of 1.5m below the existing floor, which would indicate an earlier building on the site.

"The outline suggests a pre-Norman structure," she said. "We also know that St Wilfrid, who created a monastery at Selsey, sent a priest called Eappa to the Portsmouth area to convert the locals between 681-684 AD. An internal trial excavation could answer the question of what age this structure is and whether it is related to St Wilfrid and Eappa. If so, it would be a very exciting find, as it would give us the clearest view of the history of Christianity in this city that we have ever had."

The church is now working with English Heritage, Portsmouth City Council and the Diocese of Portsmouth to determine the next steps. Another trench will be dug inside the church to investigate next Easter or next summer. Only one part of a church built by St Wilfrid and his priests - a fragment of a crypt under Ripon Cathedral - still survives to this day.

The find coincides with the launch of the St Mary's Project, a seven-part scheme to re-develop the church building to serve its community in line with its historic values.

It is likely to involve the refurbishment of St Stephen's Chapel on the south side; the provision of new community rooms; repairs to the tower; the re-development of the west end to provide office space and a welcome area; the re-ordering of the worship space at the east end; a full restoration of the historic Walker organ; and a new car park to the north of the church - between now and 2007.

The vicar, Canon Bob White, said: "The church has been serving the community here since at least 1170 and holds a place in the affections of many who live and work in the city. Many regard it as 'their' church even though they have long moved out of the geographical parish. The current building is a major landmark and its history includes two vicars at the start of the 20th century who went on to become Archbishops of Canterbury and York.

"The present congregation want to remain true to the values of the Victorians who wanted this building to be a 'silent witness to God' and a centre for the community. But we need to address its poor facilities, comply with new legislation about disabled access and make it a more welcoming place for school visits, concert audiences and others. By addressing all these issues at the same time as any archaeological work is going on, we hope to minimise any disruption."

The church completed a major restoration project to repair its roof during the 1990s, raising £1m in grants and donations for the Angels 2000 appeal. The idea is to move now from simple restoration to the creation of new facilities for the 21st century.