It became the butt of jokes and insults even before it opened in 1977, and they have continued to this day, but the Pompidou Centre is still striking an irreverent pose in the historic heart of Paris. This year, to the enduring frustration of its critics, the building hits the venerable age of 40.

The anniversary is to be celebrated with preparations for a two-year facelift expected to cost ​at least €100m. But architectural traditionalists hoping for a more sober, conventional look will be disappointed. According to plans disclosed to the Observer, the refurbished building, once described as resembling an oil refinery, will look just as inside-out as ever.

The massive renovations scheduled for 2018-20 include replacing the “caterpillar” – the famous escalators that snake up the front of the building – at a cost of €20m. But there is no plan to change the appearance of the structure. Museum officials also hope to avoid having to close during the work.

“It will be a sort of construction game, but our aim is to stay open,” said Serge Lasvignes, president of the Pompidou Centre. “That is the objective.”

Lasvignes said the original idea of the architects, led by Italian Renzo Piano and Briton Richard Rogers, to have the nuts-and-bolts inner workings of the museum on the outside, freeing up gallery space, meant much of the structure, which closed for an €88m refurbishment between 1998 and 2000, suffered from its continual exposure to the elements.

“It means that the building needs a lot of renovation,” he said. “But nothing will change outside, even though we discovered that some structures, like the large funnels that were once part of the air system, are now just decorative.”

The 2016 Magritte exhibition. Photograph: Ian Langsdon/EPA

Piano and Rogers, young and unknown in 1977, intentionally created something extraordinary on the assumption that they stood no chance of winning a building design competition that had attracted more than 600 international entries.

Speaking to the Anglo-American Press Association last week, Lasvignes revealed that conservative president Georges Pompidou, who had commissioned the building, took one look at the architects’ model and said: “Good grief, that’s going to cause a row.”

Le Monde called it “a rape … an architectural King Kong”; others said it looked like something covered in scaffolding. Even now, it divides opinion.Parisian critics still tell visitors that the best view of the city can be had from the Pompidou Centre’s rooftop terrace – now a chic and exclusive restaurant – not because you can see its most famous landmarks, but because you cannot see the Pompidou Centre.

“When the museum first opened, people said it resembled a huge department store and it was out of the question to put masterpieces in “that thing”. Even artists didn’t want their work put in a building they thought looked like a supermarket,” Lasvignes said.

“Today there are still people who contest the architecture but are seduced by our exhibitions, there are those who think it’s an architectural icon but don’t like what’s inside, and there are others who find it different and are curious to visit. I’m happy with that and we will do our best to stay true to the vision of the architects, while managing the security and other constraints we are faced with.”

While the Louvre and Musée d’Orsay reported losses last year, largely due to a drop in foreign visitors following terrorism fears, the number of people visiting the Pompidou Centre rose by 9% to 3.33 million (still well below its 1984 high of 8.41 million).

Lasvignes says this was partly due to several popular exhibitions – Paul Klee, René Magritte and the Beat Generation among them – and because the Pompidou Centre was “multi-disciplined”; it is home not just to the art museum, but also a library and a music and art research centre, making it less dependent on tourists. “After the terrorist attacks, the French made the decision not to sit at home, but to go out and enjoy the culture on offer. This is why our exhibitions were such a success. We are proud of this.”

He said the challenge was for the museum, which has 120,000 works of art, only 5% of which are on display, to avoid falling into nostalgia and to remain original and in touch with contemporary artistic and cultural movements.

Asked what he will do if the Front National’s Marine Le Pen wins the presidential election in May and slashes the culture budget, as she has pledged, Lasvignes grimaced. “Obviously it would be upsetting as director of a public establishment to find myself working with them. Unless it was impossible, I would stay in a spirit of resistance,” he said, diplomatically. “Culture is crucial for the opening of the mind to things that are different, strange, foreign … so we would have to be in opposition to any government whose programme was about closing the mind or about cultural conservatism. In that case, the Pompidou Centre and French culture would have to rediscover its militant role.”

The Pompidou Centre’s anniversary will be marked by a series of exhibitions, theatre, film and dance productions across France, including Henri Matisse in Lyon, Kandinsky in Grenoble and Joan Miró in Libourne.