The composition of the Italian parliament was reordered on Wednesday after Foreign Minister Luigi Di Maio departed from the Five Star Movement and took dozens of lawmakers with him.
Di Maio had announced he was splitting from the populist Five Star Movement following weeks of party infighting over its stance on supplying arms to Ukraine.
"Today I and many other colleagues are leaving the Five Star Movement.
"We are leaving what will no longer be the first power in parliament tomorrow,'' the 35-year-old said late Tuesday in Rome.
Di Maio and the former prime minister and current party leader Giuseppe Conte had disagreed on the issue of weapons deliveries.
While Conte recently called for no more weapons to be sent to Ukraine.
He urged the crisis to be solved through diplomatic channels, Di Maio continued to support Italy's party-independent Prime Minister Mario Draghi, who supported sending more arms to Kyiv.
Supporting Draghi made Di Maio unpopular among some elements of his own party.
Di Maio and some 50 other Five Star members had now formed the party Insieme per il Futuro "Together for the Future''.
The names of those who had switched allegiances were read out in the Chamber of Deputies, the lower house of parliament, on Wednesday morning.
The Five Star Movement had been the largest party in Italy's parliament since 2018 and was part of Draghi's multi-party government, in spite of suffering significant losses in recent local elections.
Criticism came from the co-ruling parties.
"The government must not stop because of the rupture in the Five Star Movement,'' tweeted the party leader of the right-wing Lega, Matteo Salvini.
"Nothing will change for the government,'' former prime minister Matteo Renzi, of the splinter party Italia Viva, told Rai Radio 1, adding that the government would not topple."
The Five Star Movement was now no longer the largest party in the Chamber of Deputies that is now Lega, calculated the newspapers Corriere Della Sera and La Stampa in their Wednesday editions.
In the smaller Senate, the two parties would have the same number of seats.
But this tally depended on how many politicians ultimately followed Di Maio into his new party.