A documentary about an unemployed middle aged couple in one of France's poorest towns so desperately in debt that they were on the point of burning their home has been playing to packed cinemas, with audiences cheering in the aisles.

“Merci Patron!” (Thanks boss!) may sound like a film to slit your wrists to, but it has become a hilarious rallying cry for thousands of French workers who have lost their jobs — or fear losing them — to cheaper foreign labour.

In the film, former textile workers Jocelyne and Serge Klur not only take on the “king of the catwalk” Bernard Arnault for “ruining their lives” by delocalising their jobs to Poland — they make him pay.

With the help of leftwing activist and film-maker Francois Ruffin they set up a sting operation to get the cash to save their home and land Serge a full-time job, running rings round Arnault's sidekicks, including a former intelligence officer and a politician from the ruling Socialist party.

In the process the couple, from the depressed Nord region close to the Belgium border where Arnault built his LVMH fashion and luxury goods empire, have become the country's new working-class heroes.

– David and Goliath –

Protestors demonstrating against reforms of France's labour laws have taken up the film's title as a slogan on marches and sung its “Merci Patron!” theme song — a 1970s comic skit urging bosses and workers to swap places.

The billionaire himself has remained tightlipped about the affair, with LVMH — which owns such fabled brands as Dior, Givenchy and Louis Vuitton — declining to comment to AFP.

Critics however have hailed the film as a “David and Goliath stick-up that does the heart good” with Telerama calling it a “joyous thriller… with a perfect mix of humour and social comment”.

It also tops French website Allocine's audience ratings of the best films currently on release alongside another documentary, “Demain” (Tomorrow), which showcases positive solutions to the global climate crisis.

Ruffin said the film's magic was its “liberating effect” on audiences. “When you see such an empire trembling in front of something so insignificant it has a liberating effect,” he insisted.

“People leave the cinema enthusiastic and ready to act.”

Sociologist Michel Pincon said its Michael Moore-style scenario had caught the mood of a country where “people crave a little security in a world where capitalism has become more and more unbridled.”

Pincon admitted that the movie's “audiences were mostly middle class” but the debates after screenings “prove that they too are also worried for themselves and their children”.

– Beer and stinky cheese –

The film plays strongly on France's working-class northern “Ch'ti” culture, contrasting the locals love of beer, French fries and stinky Maroilles cheese with Arnault, who owns the Moet & Chandon, Dom Perignon and Veuve Clicquot champagne houses.

John Baxter, an American who saw the film in Paris, described the atmosphere in the packed cinema as “electric”.

But he could never imagine a US business leader compensating former workers years after getting rid of them.

“Arnault is the bad guy of course — and the sting is at his expense — but he doesn't come out of it all bad. He clearly has some kind of a conscience. Most American business leaders would not give these people the time of day. They would just blow them off,” he added.

Arnault sparked the wrath of the French left in 2012 when he applied for Belgian nationality after the government proposed higher taxes on the rich, prompting the Liberation newspaper to run the front page headline, “Clear off, rich loser!”

Like its protagonists, the film faced an against-the-odds battle to get made, losing half of its tiny 30,000-euro ($33,000) budget when state funding was withdrawn “without explanation” at the last minute.

Journalists working at France's highest selling national newspaper, Le Parisien — which is owned by Arnault — claimed they were banned from writing about it.

The host of a top radio show also admitted that the film's director Ruffin was “uninvited” from his show after management intervened, apparently fearing they would lose advertising from Arnault's businesses.

But with more 120,000 people flocking to see the film on little more than word of mouth, it is now being expanded onto nine times as many screens across France.

Meanwhile, the Kenzo suits that the Klurs once made in Poix-du-Nord are now manufactured in Bulgaria after LVMH switched production from Poland after wages rose.

But salaries have fallen so sharply in crisis-hit Greece that the Bulgarian factory's owners are considering moving some operations there.