Lazio ultras have been enraged by the choice of song new full-back Elseid Hysaj made at his team initiation.
Footage was shared, and swiftly deleted, by midfielder Luis Alberto, who was laughing along with other Lazio players outside a restaurant when Hysaj stood on a chair to belt out 'Bella Ciao'.
The tune is historically seen as a left-wing anthem - the political opposite to Lazio's die-hard fans who are infamous for aligning themselves on the far right.
'Bella Ciao' was sung by the Italian Resistance Movement, who fought against the Nazis and Benito Mussolini's fascist dictatorship in the 1920s, 30s and 40s.
It has since risen to prominence after featuring prominently in the Netflix series Money Heist (Casa de Papel).
But the song is most associated with the movement that liberated Italy from fascism and the seething Lazio ultras have put up a banner on the Ponte Flaminio bridge in Rome, one of the most visible points in the Eternal City.
It reads: 'Hysaj is a worm, Lazio are fascist.'
One Lazio supporter, Franco Costantino, told Adn Kronos: 'Historically ours is a far-right fan base and we are proud of it.
'Bella Ciao sung with the Lazio shirt is something out of this world, Hysaj made a mistake. He has no excuses.'
The jovial video showed the 27-year-old Albanian defender, who recently joined Lazio on a free transfer after leaving Napoli, belting out the words while checking the lyrics on his phone before passionately shaking his fists at the climax.
Alberto could also be seen singing along, as could a number of the other players but it is Hysaj who is in the ultras' crosshairs.
According to Calciomercato in Italy, a group of ultras went to the restaurant to confront Hysaj but the team had already left by the time they arrived.
They are said to have then arranged a meeting with him, Lazio sporting director Igli Tare and a group of police officers outside the team hotel.
Whether tensions were calmed over the course of that meeting remains to be seen and Hysaj could face a fierce reception in his first game at the Stadio Olimpico.
The Albania captain joined Lazio as a free agent after his contract at Napoli expired.
Maurizio Sarri, Lazio's manager and former Chelsea boss, is said to have been unhappy that the ultras have been allowed to interfere with his players.
The club released the following statement: 'It is the job of a Club to protect its player and remove him from situations where he is being used for personal and political gain.
'In this case they certainly had nothing to do with the informal and friendly context of the incident.
'The training retreat must continue in the climate of calm that we enjoyed up until today.'