"I am proudly waiting for the next antisemitic chant. Go on as much as you like, cowards", David Guetta, an Italian radio football commentator, wrote on his blog after being the victim of a racist attack in London.

A well-known public figure in his native Florence, Guetta, who is Jewish, was in London last week for Fiorentina's Europa League match against Tottenham when he was recognised in an Underground station by a group of Fiorentina fans, who started chanting: "Guetta, a train to Mauthausen is waiting for you".

"They must have been singing it among themselves for a while, since they seemed all in tune", said Guetta.

Another sports reporter, who was with Guetta at the time, described the episode as blood-chilling and warned about its implications: "The line between words and action is getting thinner", wrote Ernesto Poesio in Florence's local paper, Corriere Fiorentino.

According to the Milan-based Observatory on Antisemitism, racism in and around Italian football stadiums has greatly increased over the past decade, targeting blacks, Jews and immigrants.

The increase in frequency of hate speech coincides with a decline in support among traditional Italian football fans. Stadiums are half-empty: only 54 per cent of seats are filled during an average Serie A game, making it Europe's least followed major football tournament according to 2015 figures. By contrast, 96 per cent of seats are filled in an average English Premier League match,

The result is that extremist supporters - often with ties with far-right movements - are now in a position to dominate proceedings and out-shout the genuine fans.