It seems like a long time since we got ourselves in a tizzy over the explicit sexual content in a motion picture. Nobody saw it coming. When, at the start of this year, the programme for the Cannes Film Festival was unveiled, Abdellatif Kechiche's Blue is the Warmest Colour did not feature prominently in many previews. The Franco-Tunisian director had won plaudits for his 2007 film Couscous – an intense, realist drama concerning a Mediterranean restaurateur – but he had nothing like the recognition factor of fellow competitors such as Roman Polanski, Steven Soderbergh or Jim Jarmusch.

A three-hour study of the relationship between a young artist and a budding teacher, the film sounded as if it might be hard work. But, as the action crept on, it became clear that we were watching something extraordinary. Few previous pictures have charted the dynamics of love with such near-scientific rigour. By the close, many punters were convinced they had seen the Palme d’Or winner. And so it proved.

“Initially I wasn’t aware of anything like that,” Kechiche says. “At that screening it was the first time I had seen it on a big screen. I was more concerned about the sound, the quality of the image and so on. I simply didn’t grasp the impression it had made.”

By the time the top prize was announced, Blue is the Warmest Colour was the runaway favourite. In order to get around Cannes' weird rule that prohibits the awarding of acting prizes to the Palme d'Or winner, Steven Spielberg's jury granted Adèle Exarchopoulos and Léa Seydoux – the lead performers – their own honorary Palmes d'Or. The two women turned up to smile. Most observers applauded another successful bash by the Riviera.

But trouble was brewing. Few people who mattered were likely to get in a kerfuffle over the relative explicitness of the lesbian sex scenes. We're past that now. Right? But the fact that the three main participants – director and two actors – were straight did threaten to kick up a few questions as regards style and emphasis. Manohla Dargis of the New York Times (a dissenter from the initial critical consensus) worried about "the way it frames, with scrutinizing closeness, the female body". Then Julie Maroh, author of the source graphic novel, popped up to offer her own criticisms of the sex sequences. "The heteronormative laughed because they don't understand it and find the scene ridiculous," she said. "The gay and queer people laughed because it's not convincing, and found it ridiculous."

Things proceeded to get a great deal worse for Mr Kechiche. The French Audiovisual and Cinematographic Union issued a report alleging that there was "harassment" on set. Exarchopoulos and Seydoux gave an interview in which they complained about the director's aggressive, obsessively meticulous approach. Each scene had, apparently, taken aeons to film. A sequence in which they slapped one another was reshot so often they felt close to nervous breakdown. "In America, we'd all be in jail. He shot with three cameras, so the fight scene was a one-hour continuous take," Seydoux said. Work on the key sex sequence had stretched over 10 days.

Kechiche argued that the nasty aftermath had poisoned his feelings about the project. He threatened to sue. He now, apparently, would prefer if the film never got released (a decision that was beyond his control).

Five months after the Cannes win, it looks as if relations have, at least, been mended with Exarchopoulos. As we make our way towards the rear of a London hotel, we pass Exarchopoulos enjoying lunch with the man from the Guardian. (In that interview, she doesn't exactly withdraw any accusations, but she doesn't rule out working with the director again either.) Adèle helloes Abdellatif merrily. He responds with a smile. We continue towards our table. But it seems that Seydoux – who plays the older of the two lovers – is still carrying on her feud with the director (and him with her, for that matter).

Let’s not ignore the pachyderm in the parlour. What does Kechiche make of all this controversy?

There is a long, long pause, while he stares at the table and gathers his thoughts. A tall, gaunt man in his early 50s, Kechiche does a fair bit of this.

“Due to the fact that the film was such a success, it allowed some people to express resentment,” he says, slightly cryptically. “I don’t know what you what you want me to talk about. Julie Maroh? The technicians? The actors? It will take a long time to answer all of them.”

A fair point. I run through Seydoux’s accusations and ask for a few observations. Another long silence. An answer begins which winds through a forest of incriminations before – 10 minutes later – reaching a glade of disenchantment.

“It is a work of art and there are different stages in that process,” he says. “When we got to the end there were positive expressions – notably from Léa Seydoux, who spoke to me saying how much she had evolved through the shooting experience. At Cannes itself she expressed how grateful she was.”

He remembers her expressing more thanks after the Palme d’Or was awarded. Moving onto the alleged volte-face, he picks up a head of steam.

“What she then expressed is in contradiction with what she had said earlier,” he says. “She was claiming insanities in a very vulgar tone. Above all, it was lies. So then the question you can ask is: where does it come from? Does she want to be in the spotlight and have this image of the rebellious actress? It is also possible that there is this human instinct of destruction when there is too much joy. You can have this desire to destroy all that.”

There’s a great deal more where that came from. All of this puts us in a difficult position. Seydoux is not here to put her side of the argument. “Thank God we won the Palme d’Or, because it was so horrible. So now it’s cool that everyone likes the film and it’s a big success,” she said in that initial interview. And Kechiche is not holding back.

“It is indecent,” he says. “And not just towards the audience, but towards humanity and society. It’s a real insult to talk about suffering when you are cherished and adored as an actor. She gets awards. She earns more for one film than an ordinary person might earn in a lifetime. Maybe people who get up and work hard at ordinary jobs can talk about suffering.”

Sadly, all the chatter about the sex sequences – and about the atmosphere on set – has come to overshadow the film’s indisputable emotional power. Brilliant on social snobbery, family politics and the messiness of ordinary lives, the film would be remarkable even if everybody kept their clothes on throughout. But Kechiche must have suspected that there would be some degree of controversy. Even those who buy the dynamics of the sex sequences and have no prudish qualms about their explicitness are sure to be slightly discomfited by their mighty length. The longest touches on 10 minutes. What the heck is going on?

“I didn’t think those scenes would upset people,” he says in his studious manner. “I am not sure they have. I think when some people ask questions about the scene, it is simply because they are not used to seeing two female bodies making love on screen. It is necessary to the film. Most people will take them for what they are.”

Fair enough. But, again, what about the sheer duration of those sequences? The mind does start to wander a little after the fourth minute of vigorous “scissoring”. Maybe, that’s the idea. The relentlessness strips away any leering eroticism. Help us out here.

“All my films have this element of puzzling scenes in terms of length, whether it’s eating scenes or sex scenes,” he says. “That’s not the norm. It’s something I do. It puzzles people. It can disturb them because they are just not expecting it.”

This sounds like so much obfuscation. But he has a point. Couscous ended with an unsettling sequence that combined an endless belly dance with shots of its hero hopelessly chasing a moped through waterside streets. One gets the sense that Kechiche rather enjoys the notion of hammering away at an event or process until it ceases to have meaning. None of this can be easy on his actors. Observers will continue to argue about the moral justification for his alleged reign of terror on set. But it can't be denied that he gets results. Never Ultramarine: Adèle Exarchopoulos and Léa Seydoux in Blue is the Warmest Colour. Below: director Abdellatif Kechichebefore did the Cannes jury hand a Palme d'Or to a lead performer. Interestingly, Kechiche spent many years as an actor. It was not until 10 years ago, then well into his 40s, that he directed his first feature. So, he must understand the demands and pressures of acting in a way that similarly harsh taskmasters such as Stanley Kubrick did not.

“It’s a passion for me to work on this,” he says. “I am fascinated by how a person raised in a certain way can give a role life and body. The art of acting is the most complete form of art. It requires a total undressing – not in the physical sense – but a bearing of your soul. It’s a creative process that demands a sort of second state from myself also. How we arrive at that is a process of research and rehearsals. It is hard to explain how you achieve that process. It really is.”

He'd better find the language. He will be explaining himself for some time to come.

yyy Blue Is the Warmest Colour opens today and is reviewed on page 11