The Italian government adopted a new electoral law Thursday that is set to favour mainstream parties over the populist Five Star Movement (M5S) at general elections expected next year.

The new law, which looks likely to land Italy with yet another coalition government at national elections due before May, was adopted by the Senate by 214 votes in favour to 61 against.

"The scam electoral law is like a slot machine," scoffed Giovanni Endrizzi, the movement's Senate floor leader.

"You press a button, you think you're choosing but there's a software that decides for you and eats your money. Here they eat your votes," he said, in reference to the fact the system will penalise the M5S even if it wins the most votes.

As individual parties, the anti-establishment M5S and ruling centre-left Democratic Party (PD) are currently running neck-and-neck in the polls, with the centre-right Go Italy (FI) and anti-immigrant Northern League dragging their feet.

But polls indicate that no single party would snap up enough ballots under the new system -- a combination of proportional representation and first-past-the-post -- without forming a coalition.

While a divided left may flounder, an FI and League partnership is expected to leapfrog into first place. The M5S on the other hand, which has vowed never to join forces with traditional parties, would be out of the race.

- 'We're ready' -

The movement has declared the new system unconstitutional and plans to appeal to Italy's President Sergio Mattarella to stop the law in its tracks, M5S sources told Italian media.

The law won the backing of the PD, FI, League and the small centre-right Popular Area (AP).

The FI party, founded and run by former prime minister Silvio Berlusconi, was like the cat that got the cream.

"We have done something positive for our country in a moment when political clashes -- particularly due to the M5S, which is only capable of protesting and not proposing -- show no sign of abating," said FI's Renato Brunetta.

"Next, in 2018, will be our return to Palazzo Chigi. We're ready," the party's lower house floor leader said in reference to the prime minister's office.

The PD, riven by internal feuds, rushed to reassure party supporters they could still win under the new system.

"No electoral law has ever managed to see those without votes triumph, or make those with votes lose," said Luigi Zanda, head of the PD in the Senate.

The law's successful adoption after months of controversy increases the probability that Mattarella will dissolve parliament at the end of 2017 or early next year at the latest, making elections likely in March 2018.

Italian lawmakers had been under pressure to act after the country was left with two different electoral systems in its upper and lower houses of parliament in 2016 following a reform defeat for former prime minister Matteo Renzi.