Pollution and old age is causing so much damage to Canterbury Cathedral that it could have "disastrous consequences", its curators have said.
More than one million people visit the cathedral every year
A global campaign to raise money for repairs to the Kent cathedral, which was founded in 597 by St Augustine, will be launched next month.
It costs more than £6m a year to run the cathedral, which is visited by more than one million people annually.
Chief executive Brigadier David Innes said trustees were being forced to act.
"Alas, time is not on our side and, if we don't start now, the rate of decay and damage being inflicted on this unique place will increase dramatically with potentially disastrous consequences," he said in a statement.
The cathedral houses what is considered to be the finest 12th Century stained glass in the UK. It also has examples of every type of Gothic architecture.
The campaign will be launched on 3 October.