Giuseppe Conte has resigned as Italy’s prime minister after blasting Matteo Salvini, the leader of the far-right League, as an “opportunist” for triggering a government crisis that could have “serious consequences” for Italy.

Conte said he would formally resign his mandate to the president, Sergio Mattarella, after the close of the debate in the Senate on Tuesday.

The outgoing prime minister said that Salvini, deputy prime minister and interior minister, had betrayed Italian citizens after pulling the plug on the party’s tempestuous alliance with the anti-establishment Five Star Movement (M5S) earlier this month.

Salvini is eager to exploit the League’s growing popularity by bringing about snap elections.

“He is only looking after his own interests and those of his party,” said Conte. “Calling on voters every year is irresponsible,” Conte said, adding that the prospect of Salvini as Italy’s next prime minister was “worrying”.

By resigning, Conte has avoided a no-confidence vote sought by the League.

The power to dissolve parliament and call new elections rests with Mattarella, who could also seek the formation of a new parliamentary majority or install a technical government. The timing of Salvini’s manoeuvre is sensitive as Italy must present its draft budget for 2020 by the end of September.

Conte said Salvini’s choices in recent weeks revealed “poor institutional sensitivity” and “a serious lack of constitutional culture”. He also criticised the minister’s use of religious symbols in his constant campaigning across Italy, describing it is as “offensive to the faithful”.

Facebook Twitter Pinterest Salvini kisses a rosary as Conte delivers his speech to the Italian Senate. Photograph: Andreas Solaro/AFP/Getty Images

Salvini kissed a rosary and retaliated in his response to Conte in the Senate, saying “I’ll ask the Madonna for protection for as long as I live”.

“I’m the only humble witness,” he added. “My country matters more to me than the comfy seats [of power]”.

He said that Italy’s most pressing problem is its low birth rate, adding that his potential government would be one that supports a €50bn budget for 2020 that focussed “on lowering taxes, the right to life…growth, investment.”

As prime minister, he would “focus on Italians, not on Merkel or Macron … I am proud, free and nationalist. Italy will be about children, who have a mum and a dad.”

If Mattarella does call new elections, then they would need to be held within 45 to 70 days.

Conte, who is usually mild-mannered, upped the ante against Salvini at the weekend, accusing him of disloyalty and being “obsessed” with closing off Italy’s ports to migrants. The row erupted after Conte refused to sign an order banning the Open Arms migrant rescue ship from docking in the island of Lampedusa.

Italy's Matteo Salvini allows migrant children to leave rescue ship Read more

Salvini was hoping his drastic move on 8 August to withdraw from the coalition would immediately collapse the government and bring about snap elections.

The League leader has since the May European parliamentary elections been seeking to capitalise on his party’s growing popularity: it is polling in first place at about 38%. Salvini’s party is much weaker in the Italian parliament following a third place showing behind M5S and the centre-left Democratic party (PD) in March 2018 elections, when it took 17% of the vote.

His strategy could be thwarted if M5S and the PD formed an alternative majority to guide Italy through the delicate budget period in the autumn. Matteo Renzi, a PD senator and former prime minister, is spearheading talks between factions of both parties. He told the Senate: “The populist experiment works well during an election campaign, less well when it comes to government.”

Nicola Zingaretti, the PD leader, is yet to embrace the potential partnership, with a source saying on Tuesday that his first choice is elections in order to avoid a move that Zingaretti described last week as a “gift to the dangerous right”. In a statement, Zingaretti said Conte’s words against Salvini are “to be appreciated … but there is a risk of self-absolution.”

Salvini has also signalled the possibility of patching things up with M5S in order to avoid the party’s tie-up with the PD. He again handed an olive branch to the M5S on Tuesday, by saying the parties could first approve a reform to cut the number of parliamentarians, as M5S wishes, before “immediately going to vote”.

In an interview with Radio 24 before Conte’s resignation, Salvini said: “What sense is a government against Salvini, with all inside? A government needs to be strong in order to get things done. Who would an M5S-PD executive represent?”