With its old-school layout, unwieldy pages and heavy doses of slapstick humor, France's Canard Enchaine weekly may have seemed especially vulnerable as the internet upended the newspaper business.

But as its "fake job" claims against presidential hopeful Francois Fillon show, "The Shackled Duck", which last July celebrated 100 years in print, has hardly lost its bite.

Although circulation has fallen to about 400,000 copies in 2015 from 500,000 in 2010, its sales have spiked in tandem with its most explosive claims. The edition detailing the money paid to Fillon's wife, dubbed "Penelopegate", quickly sold out an estimated 500,000 copies.

But Le Canard Enchaine refuses to post its reports on its website. It accepts no advertising. Its journalists are its only shareholders, and its finances are among the healthiest in the French media landscape.

And every Tuesday night, politicians send couriers to the paper's Paris offices ahead of Wednesday's public distribution, hoping they won't be the subject of its skewering reports.

Created to take an anti-war stance during World War I, the Canard Enchaine long styled itself a "satirical" paper happy to deride the foibles of the famous, before shifting its focus towards investigative journalism in the 1970s.

In French a "canard", or duck, is the colloquial name for a newspaper, while the chains refer to the heavy press censorship of the war.

For years, hardly any of its rivals could match its stream of scoops, some of them uncovering the most infamous political scandals in French history.

Even with a relatively small newsroom that relies heavily on freelance contributors, the Canard punches above its weight, with sources ranging from diplomats and judges to corporate executives and whistle-blowers.

But the paper's scoop about Fillon's wife could be one of its biggest yet in terms of fallout.

Fillon, a conservative, was leading the presidential race until the Canard revealed that his wife had been paid more than 800,000 euros ($860,000) over 15 years for a suspected fake job as a parliamentary aide.

For Greens lawmaker Noel Mamere, "Penelopegate" is an example of the meticulous investigative work that, together with its caustic tone, has earned the Canard cult status.

"The Canard never puts out a claim that could end a politician's career without checking and double-checking," said Mamere, adding: "It's the opposite of a social network."