Five days after announcing snap elections, the Greek prime minister Alexis Tsipras faces an uphill struggle to remain in power as results from Sunday’s European elections showed Greek voters living elsewhere in the EU overwhelmingly rejecting the leftist leader.

After suffering an unexpectedly heavy defeat at home, Tsipras’s Syriza party was also resoundingly rebuffed by expat Greeks, it emerged on Thursday. With 100% of the vote counted from countries across the EU, the Greek interior ministry reported that 33.9% had cast ballots in favour of the centre-right New Democracy party in contrast to 15.3% for Syriza.

The result is further proof of the herculean task now confronting the beleaguered leftists before early elections on 7 July. On Friday Tsipras convened his cabinet amid mounting speculation of a reshuffle to boost the party’s flagging popularity. The PM was forced to announce the vote, four months ahead of schedule, after Syriza lost to New Democracy by an unprecedented 9.3 percentage points – the widest margin of defeat ever in a European election in Greece.

A second round of local polls this weekend is expected to complete the crushing performance for a party of former radicals elected at the height of the country’s debt crisis on the promise to reverse internationally mandated austerity. The vote follows Syriza’s poor showing in in a first round held alongside the EU elections last Sunday.

“The vast majority of regional governors and all the big municipalities are on course to be won by New Democracy,” Elias Nikolakopoulos, a leading pollster, told the Guardian.

Asked why the electorate seemed bent on punishing the ruling party he responded: “It’s a little bit of everything; what has happened to the middle class, the [strictures of] bailout oversight, their style in power, a know-all approach and general over-assuredness.”

Greeks living in other EU countries are believed to have been particularly negatively swayed by perceived concessions over Macedonia: in exchange for the neighbouring country agreeing to change its name, the leftist government recognised a Macedonian language and ethnicity which has left many enraged. Although applauded internationally, the name-change deal was aggressively derided by the conservatives as part of concerted efforts to appeal to nationalist voters globally.

Tsipras is expected to meet Greece’s head of state, President Prokopis Pavlopoulos, on Monday to formally request the dissolution of parliament in the run-up to the snap election.

The prime minister has declared himself to be “ready for battle” pitching the fight as one between progressive forces representing “the many” and the old political establishment that brought Greece to the brink of financial collapse.

A vote for New Democracy would take the country back to “the darkness of austerity, the darkness of crisis, the oligarchs, the International Monetary Fund,” said the 44-year-old leader as he announced the decision to bring the election forward.

But short of a miracle few believe he can close the gap. Syriza lost 600,000 voters since the last general election in September 2015. If, as expected, the leftists receive a drubbing in local runoffs this weekend, analysts say the momentum will almost certainly be with Kyriakos Mitsotakis, the 51-year-old reformist leader of New Democracy. “There’s clearly a lot of soul-searching in Syriza but it’s going to be very difficult, practically impossible for them I’d say to win,” said political commentator Pandelis Kapsis.

The spectre of supporters from smaller political forces also voting for New Democracy had, he added, increased the probability of the conservatives winning a parliamentary majority – a prospect reflected this week in strident gains on the Athens stock exchange.

On the left Tsipras also faces a challenge from his former finance minister Yanis Varoufakis, whose MeRA25 party did unexpectedly well in the European election in Greece even if it narrowly missed winning a seat in the EU parliament.

No government in Greece has escaped the gruelling effects of an economic depression not seen since the return of democracy in 1974.

Like its predecessors, Tsipras’s administration has been left reeling from enforcing the very policies it had once pledged to repeal in return for bailout funds from the EU and IMF to shore up the insolvent economy.

While the leftists successfully navigated the country’s exit from eight years of bailout oversight and oversaw a noticeable reduction in record levels of unemployment, from 28% in 2013 to less than 20% this year, economic recovery has been slow.

Tsipras’s last-ditch effort to claw back support by handing out €1bn worth of tax cuts and pension bonuses appeared to have backfired as retirees, perhaps tired of promises, also turned to New Democracy.

In four years of Syriza governance the Greek middle class has been all but decimated by runaway taxes imposed as part of a bailout program negotiated in 2015 that saw Tsipras sign up to unprecedented budget surpluses of 3.5%. Mitsotakis has vowed to reduce both.

“One of the biggest reasons for Syriza’s defeat is the middle class,” said Kaki Balli, editor of the Sunday Avgi paper which reflects Syriza’s views. “Greece, unlike other European countries, has always had a lot of class mobility. Given that every Greek wants to belong to the middle class they have been lured by the promise of less taxation and those are promises that New Democracy is making.”

• This article was amended on 1 June 2019 to remove an incorrect reference to Greeks living outside the EU being able to vote in the election.