Notre Dame is still so fragile that there is a "50% chance" the Paris landmark might not be entirely saved.
Its rector has said scaffolding installed before this year's devastating fire is threatening the vaults of the Gothic monument.
Monsignor Patrick Chauvet said restoration work is not likely to begin until 2021 - and described his "heartache" at not being able to celebrate Christmas services inside the cathedral this year, for the first time since the French Revolution.
"Today it is not out of danger," he said after Christmas Eve Midnight Mass in a nearby church.
"It will be out of danger when we take out the remaining scaffolding."
"Today we can say that there is maybe a 50% chance that it will be saved. There is also 50% chance of scaffolding falling on to the three vaults, so as you can see the building is still very fragile," he said.
The 12th-century cathedral was under renovation at the time of the accidental April fire, which destroyed its roof and collapsed its spire.
One of the toughest parts of the clean-up effort is cutting down the 50,000 tubes of old scaffolding that crisscrossed the back of Notre Dame.
"We need to remove completely the scaffolding in order to make the building safe so in 2021 we will probably start the restoration of the cathedral," Mr Chauvet said.
"Once the scaffolding is removed, we need to assess the state of the cathedral, the quantity of stones to be removed and replaced."
He estimated that it would take another three years after that to make it safe enough for people to re-enter the cathedral, but that the full restoration will take longer.
French President Emmanuel Macron has said he wants it rebuilt by 2024, when Paris hosts the Olympic Games, but experts have questioned whether that timeframe is realistic.
Unable to celebrate Christmas in Notre Dame this year, its congregation, clergy and choir decamped to the Saint-Germain l'Auxerrois Church across from the Louvre museum instead.
Parishioners at Christmas Eve Mass shared sorrow about the fire, but also a feeling of solidarity.
"I remember my mother told me that she was watching TV, and that there was a fire at Notre Dame. I told her 'It's not possible', and I took my bike, and when I arrived I was crying," said Jean-Luc Bodam, a Parisian engineer who used to cross town to attend services at the cathedral.
"We are French, we are going to try to rebuild Notre Dame as it was before, because it is a symbol," he added.