When the Mayor of Buccinasco voiced suspicions about the “infiltration” of the local economy by men with Calabrian accents, his car was set on fire. When he excluded Calabrian-run building companies from public contracts in his town near Milan, he received a “Happy Easter” card – with a bullet inside it.

Maurizio Carbonera says that he will not be intimidated. But his experience is far from unique. Yesterday police arrested 20 members of the ’Ndrangheta – the Calabrian Mafia – in Milan, including the notorious clan boss Salvatore Morabito, and seized 250kg (550lb) of cocaine.

Until recently the ’Ndrangheta confined its activities to kidnappings in the remote rural South. But Milan magistrates suspect that it has taken control of illegal drugs and legitimate businesses in the north with a turnover of €40 billion (£27 billion) a year – equivalent to 3.5 per cent of Italian GDP.

Yesterday’s raid centred on the Milan fruit and vegetable market, which police said had been used to launder drugs money and distribute cocaine.

“The ’Ndrangheta controls banks, restaurants, shopping centres, construction companies, betting shops, luxury boutiques, supermarkets, night-clubs, discotheques and gaming arcades thoughout Milan and Lombardy,” says Laura Barbaini, the Milan prosecutor leading an investigation into the Calabrian Mafia’s growing grip on the Italian financial and economic nerve centre.

Less well known than the Cosa Nostra (Sicilian Mafia) or the Neapolitan Camorra, the 150 family clans that make up the ’Ndrangheta – from a Greek word meaning “honour” – have discreetly established a network of accountants, lawyers and bank managers in Milan, according to Ms Barbaini. It is estimated to have 10,000 members in Italy and abroad.

“The ’Ndrangheta has become as adept at money laundering and online transactions as it once was with sawn-off shotguns and extortion rackets in the wilds of Calabria,” says Mario Venditti, an antiMafia prosecutor in Milan.

The Calabrian mafiosi, says Giacomo Amadori, an investigative reporter, have “swapped rough rural clothes and flat caps for slick city suits”. But they retain the “blood ties” that originally knitted the clans together in Calabria. To foil investigators and phone taps, they use impenetrable dialect, and even a whistled code used by shepherds in the Aspromonte mountains around their Reggio Calabria stronghold.

The ’Ndrangheta’s move north has transformed Milan into “the cocaine capital of Europe”, investigators say, with three times as many drug users as in the rest of Italy. Last year police seized nearly 1,400kg of the drug in Lombardy.

Alberto Cisterna, a prosecutor in Reggio Calabria, says that the ’Ndrangheta’s drugs smuggling techniques are “endlessly inventive and ingenious”. “Pentiti”, or turncoats – common in the Sicilian Mafia – are almost unheard of in Calabria. The ’Ndrangheta also keeps a low profile, avoiding violence – although investigators say that it is stockpiling weapons in case of “turf wars” with other criminal gangs.

A third of the ’Ndrangheta’s illicit profits are ploughed back into crime, but two thirds are invested in “legitimate” businesses, according to Francesco Forgione, the head of the Italian parliamentary antiMafia commission. The logical next step may be for the ’Ndrangheta to dominate the politicians themselves.

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