The only known script for Jean-Luc Godard’s seminal New Wave film Breathless (À Bout de Souffle) will be auctioned later this year after coming to light for the first time in more than 60 years.

About 70 pages of Godard’s handwritten notes and synopses of some of the most famous scenes, including the movie’s dramatic opening, were discovered in the estate of the celebrated producer Georges de Beauregard.

Breathless, which follows the doomed affair between an American student in Paris (Jean Seberg) and her hoodlum boyfriend who is wanted for gunning down a police officer (Jean-Paul Belmondo), is a keystone of France’s Nouvelle Vague movement that shook the cinema world, including Hollywood.

Godard’s innovative method of working means that scripts of his films are rare. He shunned formal scripts and liked to write dialogue the night before a shoot, to encourage actors to behave naturally. He also had a penchant for destroying written records.

Anne Heilbronn, the head of books and manuscripts at Sotheby’s Paris, which is auctioning the manuscript along with photographs and other items from De Beauregard’s estate, admitted she was overcome with emotion when she saw the documents.

“I wanted to cry. It was an incredible shock to actually have this manuscript that is a record of the history of French and world cinema in my hands,” she said.

“À Bout de Souffle is an iconic film for the whole world and here we see part of the dialogue, the scenes, the trailer, for the first time since 1960. As far as we know, it is the only script of its kind.”

The original outline for the story, based on a news event that enthralled France in 1952, had been written by Godard’s friend and fellow New Wave director François Truffaut, who allowed him to develop the plot.

De Beauregard, another of the movement’s key figures, had met Godard through Truffaut, and made a leap of faith in agreeing to produce Breathless, the then unknown director’s first full-length film.

Godard approached the shoot over the summer of 1959 with a documentarist’s method, filming in the streets with a handheld camera and mostly with natural light. Having started with a precise screenplay for the first 14 minutes of action, he ditched it and decided to write each day’s script the night before. As the dialogue was to be synchronised in post-production, he did not mind if the actors forgot lines they had often been given on the morning of the shoot, as they frequently did.

Sotheby’s says it is describing the manuscript as “partial”, not because anything is missing but because Godard did not submit a full synopsis and script to the ministry of culture’s National Centre of Cinematography and Animated Pictures, as would have been normal at the time, and made much of it up as he went along.

Godard explained his thinking in Cahiers du Cinéma in 1968. “I had written the first scene [Jean Seberg on the Champs Elysées] and, for the rest, I had a huge number of notes corresponding to each scene. I said to myself, this is outrageous! I stopped everything. Then I thought about it … instead of finding something a long time before, I’ll find it just before. When you know where you’re going, it should be possible. It’s not improvisation, it’s last-minute fine-tuning.”

In 1967, Truffaut wrote: “The passing years confirm our certainty that À Bout de Souffle will have marked a decisive turning point in the history of cinema, as Citizen Kane did in 1940. Godard has shattered the system, he has made a mess of the cinema.”

The manuscript, with an estimate of £350,000-£500,000, will be sold as part of Sotheby’s online auction of books and manuscripts, open for bidding on 14-18 June. The single lot will include four original photographs of Godard and Seberg, a vintage contact sheet, and letters from Godard and the actor and director Roger Hanin, who appeared in the film, all from De Beauregard’s archive.

“As someone who adores the cinema and is passionate about it, I can say it is one of the best film scripts I have ever held,” Heilbronn said.