St James' Priory dates back to 1129
Restoration work will begin on St James Priory in central Bristol including essential repairs to masonry and the stained glass windows.
The Grade I listed church, which is on English Heritage's Buildings at Risk Register, suffers from a leaky roof and rising damp.
The cost of the work is estimated at £4m and will take 18 months to finish.
The official start was marked by 100-year-old parishoner Phyllis Townsend handing the Priory key to the builders.
Work will also be carried out to improve the outer north aisle and east end rooms to provide space for meetings, concert facilities and a cafe.
Susan Jotcham, project director said: "The start of repairs, conservation and development work on St James Priory is something that we have been dreaming would happen for years.
"This is such a special day for us, none of this would have been possible without the wonderful generosity of The Heritage Lottery Fund, English Heritage, National Churches Trust as well as other local and national trusts and donors.
"St James Priory has been a haven for Bristolian's for almost 900 years, what was so nearly a ruin can now can look forward to a wonderful future."
The Priory was founded in 1129 by Robert, First Earl of Gloucester, and is Bristol's oldest standing building.
While the work is ongoing the church will be closed for daily worship but will remain open for the celebration of Sunday Mass.