An offshore platform a stone's throw from Dante's beach on Italy's Adriatic coast has become the focus of a politically charged national referendum on the country's oil and gas drilling concessions.

The debate pits environmentalists against the government and big business, and revolves around a change in the law in January that ruled existing concessions within 12 miles (19 kilometres) of the coast should last until the fields are depleted.

Prime Minister Matteo Renzi's centre-left party, keen to be seen as pro-business, has called on Italians not to vote on April 17. That has infuriated ecologists who insist platforms near the shore present health risks and devastate protected habitats, and should not be given an extra lease of life.

From the threat to a prized mussels industry to fears coastal erosion will eventually see an encroaching tide swallow up nearby Ravenna, with its Byzantine mosaics and the revered poet Dante Alighieri's grave, the row has Italians in a tizz.

A "yes" vote to reverse the law "would send a clear signal that Italy wants to embrace renewable energy," Giulio Kerschbaumer, 34, head of environmentalist lobby Legambiente in the Emilia-Romagna region, told AFP.

Pointing to a recent scandal in Italy that saw a top minister resign over alleged favours to French oil giant Total, Kerschbaumer said Legambiente believed the law had been changed "as a present to the oil companies".

- Split deepens -

The vote, called by regions largely led by Renzi's Democratic Party, has deepened a split within the centre-left and is being seen as a key test for the PM ahead of a constitutional reforms referendum in October.

The 41-year-old has bet his political career on the reforms and many see this vote as a pretext to attack his leadership.

Nine regions asked for and are affected by the referendum, from Basilicata, to Calabria, Sardinia and the Veneto. Most observers say the result will have little effect on the government's coffers either way: concessions within the 12-mile band brought in a relatively modest 38 million euros ($43 million) in royalties in 2015, according to official data.

At the heart of the drama lies Ravenna, once the capital city of the Western Roman Empire, and now the hub for oil and gas across the country's centre and north. Some 47 of the 92 platforms in question lie here in the Emilia-Romagna region and the local economy is inextricably tied to the sector's fate.

According to the Filctem Cgil union, closing rigs when the concessions expire would put at least 2,500 jobs on platforms and in services at risk. A report this year by the Nomisma research foundation found there were some 140,000 people in the region whose jobs depend on oil and gas.

Gianluigi Bambini, who runs a maritime transport company, said business had already been hit by stagnating output.

"We had 264 employees 18 months ago, now we have 180, most of those let go are young," he said as one of his crew boats pulled up next to an Eni rig, shrouded in mist, with hundreds of gleaming mussels clinging to its red tubular legs.

- Rising seas -

Plucked from the platform by divers and steamed in vats with tomatoes, onions and parsley, freshly gathered molluscs are served up by supporters of the "No" camp in the port area of Marina di Ravenna to raise support.

Giovanni Fucci, president of a local mussels farming association, is fighting to restore the sector's name after a Greenpeace report last month asserted molluscs from Eni platforms contained traces of heavy metals and hydrocarbons.

"Every 15 days health inspectors come out to sea and take samples, they analyse them and rule them fit for human consumption. These mussels are sought by the best Italian chefs for their taste and quality," he said, adding that the market is worth 126 million euros a year in Emilia-Romagna alone.

Back on Dante beach, local campaigners rejoice that the referendum is drawing attention to coastal erosion they claim is caused by nearby rigs, the closest of which lies just 1.5 kilometres offshore.

Pine trees in a national nature reserve set back from the beach are dying, according to 74-year old Pasquale Minichini, because "rising sea water is killing them at the roots" -- aggravating a phenomenon caused by global warming.

He cites research that says Ravenna could return to its Roman-era state -- small islands in a lagoon, like Venice -- by 2100.