The former Italian prime minister Matteo Renzi has quit the centre-left Democratic party (PD) as he carves out a political comeback that risks sabotaging the new ruling coalition.

Renzi, who engineered the alliance between the PD and the anti-establishment Five Star Movement (M5S) in order to oust the far right from government and stave off snap elections, said he was leaving the party to form a new centrist force that would “do politics differently”.

“The victory we obtained in parliament against populism and [the former deputy PM Matteo] Salvini was important to save Italy, but it’s not enough,” Renzi wrote on Facebook. “Now is the time to construct a house that is young, innovative, feminist … where ideas for Italy and Europe are launched. There is an enormous space for different politics.”

Renzi’s move, which was widely anticipated, comes a week after the uneasy coalition won a vote of confidence in both houses of parliament.

Renzi said parliamentary groups for the new force would be created this week and that 30 PD lawmakers were expected to join him immediately. He told the prime minister, Giuseppe Conte, he still backed the new coalition and that his move would increase support for it among the electorate.

Renzi was forced to resign as prime minister in December 2016 following an embarrassing defeat in a referendum on constitutional reform. He then stepped down as leader of the PD after its dismal performance in general elections in March 2018. Those elections led to the creation of a tempestuous alliance between M5S and the far-right League.

Salvini, former interior minister and leader of the League, plunged Italy into political turmoil in August after pulling the plug on the government as he sought to exploit his popularity and trigger snap elections. In order to thwart the possibility of a fully far-right government, Renzi engineered talks between an initially reluctant PD and M5S.

“Sending Salvini home will remain one of the proudest things on my CV,” Renzi told La Repubblica.

Nicola Zingaretti, who has revived the PD in the polls since being elected leader in March, said it was an “error” to split the party. “Especially at a time when its strength is indispensable for the quality of our democracy,” he added on Facebook.

The move will make Conte’s job more challenging and complicate policymaking owing to the demands of three different political forces. It is unclear what Renzi’s long-term aim is. He told the Observer in August that his idea of a PD-M5S coalition was to “save Italy” and not reignite his own career.

“He sees this alliance moving too much towards the left and missing a big chunk of the electorate who do not identify with it … he could grab that 10% and push the system more towards the centre,” said Franco Pavoncello, a political science professor and president of John Cabot University in Rome.

“I don’t think he would go for early elections … he will try to keep the government going for at least another couple of years.”

Renzi has one of the lowest approval ratings among Italian politicians, and pollsters predict his new party could initially capture 3-5% of the electorate’s support.

“Renzi is unpredictable and his bold moves in the past have been badly defeated … so we have to see what happens this time,” added Pavoncello.