St Mary’s Church in Devizes, one of the most historic buildings in the country, is likely to close if permission is refused for an extension, say campaigners.

Supporters of the church say if it closes it will end up dilapidated like the Assize Courts in Northgate Street, which has been standing empty since its closure in the 1980s.

The 12th-century Grade I listed church was declared redundant by the Parochial Church Council in 2006 due to its dwindling congregation but the PCC was asked to reconsider by the Church Commissioners.

Since then the PCC has carried out extensive consultation and received support for its plans from community groups to build a curved glazed cloister extension on the western side of the church.

The extension would provide an office, two meeting rooms, kitchen, toilet facilities and a vestry while the inside of the church would be used for performances and exhibitions with racked seating providing capacity for 260 people.

The planning application for the extension will be determined tonight by Wiltshire Council’s Eastern area planning committee but the scheme is recommended for refusal because planning officers believe the extension would be an alien feature and cause “substantial harm” to the church.

Planning officer Karen Guest says St Mary’s is among the most important of Grade I listed buildings in the country.

English Heritage has objected to the extension as has Wiltshire Council’s conservation officer.

Reverend Canon Paul Richardson, rector of St Mary’s and St John’s Churches, said the extension will ensure the future of St Mary’s by enabling wider community use and if permission is refused there is little prospect of the church remaining open.

“There is a very real risk if we can’t get planning permission to redevelop St Mary’s that it will be closed and it could fall into disrepair.

"There is no Sunday congregation at St Mary’s. We have one service a week on Tuesday evenings with about half a dozen people attending.

“If planning permission is refused we don’t just lose the benefit of a wonderful building in the town but also the opportunity of providing a space for community performing arts and exhibitions, which is wanted and needed in the community.

“Since the PCC decided not to close St Mary’s in 2006 we have replaced the roof at the eastern end of the church, the nave roof and repairs inside. None of that would have happened had it closed. I would be very, very sad if St Mary’s closed and its condition deteoriated.”

The extension would cost £1.2million and about £300,000 has been raised so far. Mr Richardson said: “We are very confident the money will be found, we are not concerned about the cost.

“We have more money pledged but it is dependent on receiving planning permission. If we get planning permission we would be looking to begin the work on the extension late spring, early summer next year with it being up and running by Spring 2015.”

The planning committee meeting is at 6pm tonight at Devizes School.