ROME (Reuters) - Three senators of the ruling anti-establishment 5-Star Movement quit the party on Thursday to join the far-right League opposition group, cutting the government’s already slim majority in the upper-house of parliament.

The move, which was announced by League leader Matteo Salvini, will make it harder for Prime Minister Giuseppe Conte to govern an already fractious coalition and leave him vulnerable to opposition ambushes in the 321-seat Senate.

The walkout by Ugo Grassi, Stefano Lucidi and Francesco Urraro also underscores deep divisions and unhappiness within the ranks of 5-Star, which emerged as Italy’s top party in 2018 national elections but has seen its support halve in recent months as it struggles to get to grips with difficult policy decisions.

"The League has opened a cattle market. It should publish its price list so we can see how much a parliamentarian costs per kilo,” the 5-Star’s embattled leader, Luigi Di Maio, wrote on Facebook.

Salvini has previously rejected suggestions that he might try to poach other party’s parliamentarians in an effort to bring down the government and trigger early elections that opinion polls suggest he would easily win. On Thursday he dismissed suggestions he was buying support.

"We don't have a price,” he said in a statement.

The trio’s departure leaves the coalition on paper with just a 5-seat majority in the Senate, although a number of senators from some small parties, that are not in the government, are likely to keep backing Conte in crunch votes to avert political chaos.

The senators swapped sides after the parliament approved on Wednesday a coalition resolution on a reform of the euro zone bailout fund (ESM). They complained that parliament had been sidelined during discussions on the highly technical mechanism.

The League leads the opposition against the government of the 5-Star and the centre-left Democratic Party, that was formed in August after Salvini walked out of his coalition with the 5-Star in a failed attempt to provoke an early election.

The government won its initial, obligatory vote of confidence in the upper house 169-133 thanks to the backing of some independent senators.

(Reporting by Angelo Amante; Editing by Lisa Shumaker)