Five years after being ravaged by fire, Notre-Dame Cathedral has returned to its former splendour months ahead of its planned reopening, participants in a recent visit to the monument said.

The fire at the UNESCO-listed cathedral, which used to welcome 12 million visitors a year, shocked the world on April 15, 2019.

But now, the inside of Notre-Dame is at its most luminous in living memory, visitors said.

"It is wonderful to see these colours that had completely disappeared," said Guillaume Normand, vice rector of Notre-Dame, as he inspected the completely restored chapel. "Stunning," he told AFP.

When the public returns to Notre-Dame in December they will get an "unequalled perception of its dimension", added the cathedral's rector, Olivier Ribadeau Dumas. He said he was "humbled" in the face of "those who created, preserved or saved it, and those who are now restoring it".

Ongoing work is on track to meet the December deadline for re-opening, the head of the reconstruction said last month.

The monument already had a key moment in February when scaffolding came off around its spire, which authorities say will be fully visible by the time the Paris Summer Olympics kick off in July.

The spire has been covered in lead, a material that has caused much debate because of its potential toxicity.

- $900 million of donations -

In December, the cathedral regained its great cross, and got a new golden rooster to replace one that had been destroyed in the fire.

Initially, President Emmanuel Macron promised the building would be fully restored by the time the Olympics open, but the date was pushed back after restoration work hit several snags.

Authorities have still not determined the cause of the fire, although they believe it was started accidentally.

A fund-raising drive launched within hours of the fire has attracted donations of 846 million euros ($903 million).

Restoration work has been constant since 2019, except for a few weeks during the Covid crisis.

All key challenges of the restoration had now been met, said Philippe Jost, president of the Rebuilding Notre-Dame de Paris public body. The rebuilding of the nave, using wood from around 1,000 trees specially selected from French forests, had been among the toughest tasks, Jost said.

Some 250 companies and hundreds of craftsmen, architects and other trade professionals have been involved in the restoration.

The cathedral's organ, undamaged by the fire but covered in lead dust, has been fully cleaned, although it will take six months of harmonisation before its 8,000 pipes recover their full sound potential.

Natural light inside the cathedral is at its brightest in living memory after the cleanup, Jost said.

The roofing over the nave, choir and spire are among jobs still to be completed by the summer, as are floor and furniture restorations.

Starting in the autumn, the cathedral's grounds and entrance areas are to be cleared for outside work to begin.

France has just called for bids for the creation of modern stained glass for Notre-Dame, with deliveries expected in 2026.