A BuzzFeed News investigation which accused the 'anti-establishment' party of being behind a network of blogs and websites spreading propaganda and fake news, based on analysis which showed links between supposedly independent sites and the party's leadership.
When contacted by the news site, an M5S spokesperson had said the party would be unable to provide "feedback" until after Sunday's referendum.
However, the party's leadership clearly had a change of heart as BuzzFeed's investigation was reported by Italian dailies including Il Corriere and La Repubblica, the latter saying that the article "says the same thing that's been widely revealed by newspapers and books".
"BuzzFeed's inquiry is fake news," M5S leader Beppe Grillo tweeted around midday on Wednesday.
The leader also published a blog post, attacking the accusations that the party created pro-Kremlin propaganda. He attempted to deflect blame by accusing "traditional newspapers" in Italy of publishing fake news and pro-Renzi propaganda.
"The M5S has its official social media accounts, its official site and blog," Grillo said. "Other sites or social media accounts are not attributable to the M5S."
However, BuzzFeed found that Grillo's blog, the party's official websites, other news sites - including TzeTze, a supposedly independent site with over 1.2 million Facebook followers which frequently cites pro-Kremlin site Sputnik - "all share IP addresses, as well as Google Analytics and AdSense IDs".
"Another hoax reported by BuzzFeed is that M5S MPs have to sign a contract with Casaleggio Associati," Grillo said.
Casaleggio Associati is a technology firm created by M5S co-founder Gianroberto Casaleggio. The company, run by Gianroberto's son Davide following his father's death earlier this year, also runs TzeTze and La Cosa, another site claiming to be independent.
"The two journalists who wrote the article [Alberto Nardelli and Craig Silverman] should immediately correct at least this sentence and apologize to readers for having spread this false information which would only have required a moment to verify, just like the rest of the article," Grillo wrote.
BuzzFeed cited Italian newspaper Il Fatto Quotidiano which said candidates who refused to sign a contract faced a fine of €150,000. A copy of the contract for M5S candidates published by Beppe Grillo on his blog confirms that such a fine exists - however, the contract is not explicitly linked to Casaleggio Associati. Instead, it refers to a communications team led by Grillo and Gianroberto Casaleggio.
The contract states that all prospective candidates must sign the code of conduct before voting, and that violation of the M5S principles may result in expulsion from the party and a fine of "at least" €150,000.
This is just one of the scandals regarding the party's close ties with the firm to have hit M5S following its recent success in local elections. M5S is also accused of using €1000 of Senate money each month to pay for the accommodation of Casaleggio Associati employees.
Grillo also hit out at a ‘La Stampa' article published last month, which noted the close ties between Grillo's party, the network of news sites, and some of Putin's close associates. An English language version of that article can be found here.