A leading British architect who helped restore Windsor Castle after it was a ravaged by fire has said a shortage of building specialists could impede Emmanuel Macron’s target of rebuilding the Notre Dame within five years.

As bells rang across France in commemoration of the blaze, a French government official warned the Paris landmark would have burned to the ground on Monday night in a "chain-reaction collapse" had firefighters not moved as rapidly as they did.

A fund set up to rebuild the fire-ravaged Notre Dame Cathedral in Paris was fast approaching €1bn (£870m) as donations continued to pour in from billionaires, corporations and ordinary citizens.

The French president wants the cherished cathedral returned to glory in time for the Paris Olympics in 2024 following Monday’s devastating blaze, but some have already cast doubt on the ambitious deadline.

Francis Maude, director at the Donald Insall Associates, told The Independent it would be difficult, but he believes it can be done.

Notre Dame fire: What was lost and what was saved Show all 13 1 /13 Notre Dame fire: What was lost and what was saved Notre Dame fire: What was lost and what was saved Statue of Saint Denis Notre Dame https://commons.wikimedia.org/wiki/File:Paris_-_Cath%C3%A9drale_Notre-Dame_-_Portail_de_la_Vierge_-_PA00086250_-_003.jpg Thesupermat/CC-BY-SA Thesupermat/CC-BY-SA Thesupermat/CC-BY-SA Notre Dame fire: What was lost and what was saved "Close up of Virgin Mary and Baby Jesus statues at Notre Dame Cathedral in Paris, France. Other images of the Notre Dame Cathedral:" gregobagel Getty Images/iStockphoto Notre Dame fire: What was lost and what was saved Safe: Rose windows Among the most famous architectural features of the Gothic masterpiece, the stained glass rose windows are treasured artworks. The three rose windows, which date back to the 13th century, adorn the north, south and west facades. There were hopes the windows had escaped being destroyed by the fire after firefighters stopped its spread. Photos the following morning suggested the circular window of the nave had remained intact. AFP/Getty Notre Dame fire: What was lost and what was saved Destroyed: Irreplaceable wooden roof Crowds of tourists and Parisians watched in horror as the spire and roof of Notre Dame cathedral came crashing down to the ground. EPA Notre Dame fire: What was lost and what was saved Safe: Descent from the Cross The cross and statue on Notre Dame's high altar - sculpted by Nicolas Coustou in 1723 - were believed to have survived in tact despite being surrounded by smoke and debris. Reuters Notre Dame fire: What was lost and what was saved Safe: Great Organ With nearly 8,000 pipes, some dating back to the 1700s, Notre Dame's master organ is one of the largest in the world. The monumental instrument, the largest in France, was fully restored in 2013 with each pipe cleaned. Paris' deputy mayor, Emmanuel Gregoire, said the instrument remained intact following the fire. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Bells Housed in the two western towers, Notre Dame's bells have rung out at key moments in France's history. Emmanuel, the largest bell, was lifted into the south tower in 1685 and weighs over 23 tonnes. The fire was prevented from spreading to the bell towers. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Crown of Thorns Believed to be a relic of the wreath of thorns placed on the head of Jesus Christ at his crucifixion, the object was stored in the cathedral's treasury. French King Louis IX brought the relic, which is contained in an elaborate gold case, to Paris in 1238. The Crown of Thorns was saved and has been taken into safekeeping. AP Notre Dame fire: What was lost and what was saved Safe: Tunic of Saint Louis The tunic allegedly worn by Saint Louis IX as he brought the Crown of Thorns to Paris was kept inside the cathedral. It has been saved. AFP/Getty Notre Dame fire: What was lost and what was saved Safe: Statues on the roof The week before the fire, religious statues set atop the cathedral were removed for the first time in over 100 years as part of the restoration project. A 100-metre-high crane lowered the copper statues representing the 12 apostles and four evangelists onto a lorry. AFP Notre Dame fire: What was lost and what was saved Destroyed: Spire Crowds of tourists and Parisians watched in horror as the spire and roof of Notre Dame cathedral came crashing down to the ground. EPA/I Notre Dame fire: What was lost and what was saved Unknown: Choir screen It is not known what happened to the detailed choir screen at Notre Dame cathedral. AFP/Getty Notre Dame fire: What was lost and what was saved Some of the artworks from Notre-Dame sheltered as the fire devastated the cathedral AFP/Getty

“It’s ambitious, but I think it’s achievable if things go well,” said Mr Maude. “One of the things that makes me think it can be done [in five years] is the restoration of Windsor Castle. It shows projects of considerable magnitude can be undertaken in that sort of time frame.”

However he warned that the “specialist labour force” needed for such a complex and delicate operation was in short supply, and delays getting the right people could hamper the project.

“What you need is an expert project manager who would be able to identify the materials that are in restricted supply,” he explained. “The same applies to specialist labour force which is needed for the stone carving, the lead on the roof, repairs to the stain glass windows and for the internal decorations and other finishes.”

“There are a number of other big, high-profile projects which will take a large part of the labour supply needed, the specialist craftsmen – there’s the restoration of the Palace of Westminster and of the Canadian parliament in Ottawa. It’s going to place a certain demand on the skilled specialists that there are."

Westminster Abbey bells chime to mark 24 hours since Notre Dame fire started

Mr Maude added: “One would not expect people to down tools, as it were. If they’ve already been engaged in one project, you would expect them to be committed to that project.”

The warning came as the French government announced an international design competition to replace the cathedral’s iconic spire, destroyed during the fire.

Edouard Philippe announces competition to replace Notre Dame spire (AP)

Prime Minister Edouard Philippe suggested on Wednesday that architects may be able to completely remodel the spire – added to the cathedral in the 19th Century as part of renovation project led by French architect Eugene Viollet-le-Duc.

“The international competition will allow us to ask the question of whether we should even recreate the spire as it was conceived by Viollet-le-Duc,” he said. “Or if, as is often the case in the evolution of heritage, we should endow Notre-Dame with a new spire.”

Even Mr Philippe acknowledged that it would be difficult to restore the whole building in just five years. “This is obviously an immense challenge, a historic responsibility,” Mr Philippe said.

Prominent French conservation architect Pierluigi Pericolo told Inrocks magazine it could take triple that time. “No less than 15 years ... it’s a colossal task,” Mr Pericolo said.

Mr Pericolo worked on the restoration of the 19th century Saint-Donatien basilica, which was badly damaged by fire in 2015 in the French city of Nantes. He said it could take between “two to five years” just to check the stability of the massive cathedral that dominates the Paris skyline.

“It’s a fundamental step, and very complex, because it’s difficult to send workers into a monument whose vaulted ceilings are swollen with water,” Mr Pericolo added. “The end of the fire doesn’t mean the edifice is totally saved.”

Debris inside the cathedral after the 12-hour battle to extinguish an inferno that claimed its spire and roof (AP)

French companies Total and L’Oreal have promised to each donate €100m (£87m) euros to the restoration, while the billionaire families who own LVMH Group, Kering and L’Oreal pledged a combined total of €500m (£435m).

The French government is gathering donations and setting up a special office to deal with offers of assistance.

Despite extensive damage, many of the cathedral's treasures were saved, including Notre Dame's famous rose windows, although they are not out of danger.

Paris firefighters' spokesman, Gabriel Plus, said that even though they are in good condition, a "threat" continues to the gables, or support walls, because of the heavy stone statues perched on top of them.

Remarkable footage shows firefighters battling Notre Dame flames

"The roof no longer holds (the gables) up. They are holding up all by themselves," he said, adding that some statues must be removed to lessen the weight on the gables.

Scaffolding that had been erected for a renovation of the spire and roof must also be properly removed because of its weight and because it is now "crucially deformed", he added.

The cathedral is still being monitored closely by firefighters and experts to determine how much damage the structure suffered and what needs to be dismantled to avoid collapse.

Notre Dame before and after Show all 19 1 /19 Notre Dame before and after Notre Dame before and after The cathedral with the spire standing tall (left) and (right) after the fire AFP/Getty Notre Dame before and after As viewed from the Montparnasse Tower before the fire AFP/Getty Notre Dame before and after As viewed from the Montparnasse Tower after the fire AP Notre Dame before and after The heart and transept before and one day after the fire AFP/Getty Notre Dame before and after The nave of the cathedral before the fire Alamy Notre Dame before and after The nave of the cathedral after the fire Reuters Notre Dame before and after The rose window in the cathedral before the fire AFP/Getty Notre Dame before and after After the fire a hole has been exposed in the roof above the window Twitter Notre Dame before and after The cathedral from the Seine before and after Reuters Notre Dame before and after Inside the cathedral before the fire Reuters Notre Dame before and after The burning roof has left a hole in the nave Reuters Notre Dame before and after The nave after the fire was extinguished AP Notre Dame before and after Inside the cathedral after the fire Reuters Notre Dame before and after A view from the Seine of the cathedral before and after the fire Reuters Notre Dame before and after An aerial shot of Notre Dame before the fire AFP/Getty Notre Dame before and after An aerial shot of Notre Dame during the fire shows the wooden roof burning and collapsing AFP/Getty Notre Dame before and after Tourists gather near the cathedral and then seen afterwards Reuters Notre Dame before and after The spire of the cathedral is shown before and during the fire AFP/Getty Notre Dame before and after A scorched section of the exterior after the fire Reuters

A plan to safeguard the masterpieces and relics was quickly put into action after the fire broke out. The Crown of Thorns, regarded as Notre Dame’s most sacred relic, was among the treasures quickly transported after the fire broke out, authorities said. Brought to Paris by King Louis IX in the 13th century, it is purported to have been pressed onto Christ’s head during the crucifixion.

The cathedral’s famous 18th century organ that boasts more than 8,000 pipes also survived. Some of the paintings and other art works are being dehumidified, protected and eventually restored at the Louvre.

Notre Dame’s rector, Bishop Patrick Chauvet, has said he would close the once-functioning cathedral for “five to six years” acknowledging that “a segment” of the near 900-year-old edifice may be gravely weakened.