A hail of enthusiastic tweets followed the Cannes premiere of Blue is the Warmest Colour – elevating it to the status of the critics' favourite of the festival, and not a moment too long at three hours.

It also happens to contain the lengthiest, most intimate and most graphic lesbian sex scenes in mainstream cinema history. Praised for its tenderness and intensity, it has been hailed as a landmark in cinematic depictions of lesbian love and female sexuality.

Both lead actors spoke of their trust in director Abdellatif Kechiche over the four-month shoot for the film, including the scenes that, in the opinion of the Hollywood Reporter, "cross the barrier between performance and the real deal". According to Léa Seydoux, who plays the older of the two women, "I succeeded in forgetting that a camera was there."

It was a process so intense, and resulting in so much material, "that he could have made a whole lot of other films" with just the rushes, according to Adèle Exarchopoulos, who plays the younger of the two women. According to Kechiche, they regarded the filming of the sexual sections as "a game".

"We also had a great deal of fun," he said. "The actors felt they were enjoying themselves – while playing a part, of course."

Kechiche's last film, Black Venus, about a 19th-century black South African woman who was exhibited at fairgrounds, was deemed too harrowing and provocative for American and British distributors and so was never released in the UK.

The director, best known for his 2007 film, Couscous, said he would be willing to contemplate some cuts in Blue is the Warmest Colour to allow the widest possible audience to see the work. "We wouldn't want the film not to be screened because of one scene," he said, "but of course that wouldn't apply if it were the whole thing". It is, he said "a question of respecting other people's film traditions. In the States there are different ways of portraying love, sex and even violence".

Executive producer, Vincent Maravel, confirmed that the film had already sold American distribution rights "and we didn't talk about cutting anything out".

The intimate physical scenes come as only one element of a deep study of the relationship between the two young women as it grows from young first love into domesticity. Exarchopoulos, in an already highly acclaimed performance, plays a schoolgirl, also called Adèle, who embarks on a relationship with a boy, Thomas. But she finds herself drawn to Emma, a woman with blue-dyed hair whom she has seen in the street, played by Seydoux.

The film was screened less than a week after gay marriage was legalised in France. According to Kechiche, "When I decided to tell this story the particular political context did not exist – we didn't make the film to comply with a given political context. I didn't want to make a militant film that had a message to deliver about homosexuality, but of course it can be seen from that angle, and that doesn't bother me."

Unlike so many coming-out stories, there is no traumatic scene of rupture from their parents as the girls' families take on the implications of their sexuality. "I didn't want a major clash or a huge separation," said Kechiche of the story, which is loosely adapted from a graphic novel by Julie Maroh. "What I loved aside from the love story was the fact that this person missed their train, meets this woman, and her life totally changes: this meeting held out such tremendous promise. The idea that you meet someone by chance and it changes your life for ever. I was deeply touched by that idea."