Israel’s former centre-left prime minister Ehud Barak has made an election pact with leftwing politicians in an attempt to unseat the incumbent PM, Benjamin Netanyahu, in upcoming elections.

The 77-year-old retired from politics in 2013 but returned in dramatic fashion this summer to help end what he said was Netanyahu’s “rule with the radicals, racists and the corrupt”.

Barak once commanded Netanyahu in the army but later served as his defence minister. He has been a vocal critic of the prime minister, who faces three major corruption scandals and has been slammed for allying with far-right political figures.

On Thursday, Barak, another prominent Israeli politician, Stav Shaffir, and the small anti-occupation Meretz party signed an agreement to run together in the 17 September poll under a single party called the “Democratic Camp”.

Mergers are common in Israeli politics, giving smaller parties that might not cross an electoral threshold a better chance of entering parliament as an alliance. Barak does not appear to be a candidate to replace Netanyahu himself. Nitzan Horowitz, the leader of Meretz, will lead the Democratic Camp.

Another Meretz lawmaker and former party leader, Tamar Zandberg, called the merger a “dramatic move to strengthen the left” and a “significant boost to justice and equality as an alternative to the corrupt and messianic right”.

It is unclear if centre and leftwing parties will form a large enough bloc to unseat the right’s grip on power. Barak, whose premiership ended after the outbreak of a Palestinian uprising in 2000, is a wildcard in 2019 but may not tip the balance.

Israel is holding elections just months after an April poll in which rightwing parties came out with a clear lead. Regardless, Netanyahu was unable to form a majority coalition government due to infighting between religious and secular parties.

Rather than give his rivals a chance to forge a government, the prime minister’s Likud party instead pushed to dissolve the Knesset, Israel’s parliament, triggering repeat elections.

On Tuesday, Barak apologised for the killing of 13 Arab protesters by Israeli police at the start of the second intifada under his watch.

It was widely seen as a move to woo leftwingers, such as Meretz supporters, and Palestinian citizens of Israel – who make up roughly a fifth of the country’s population.

While army credentials give Barak gravitas among security-minded voters, the opposition is already filled with former military figures leading to fears his entry to the race could split the anti-Netanyahu vote. The centrist Blue and White party, which drew even with Likud in the 9 April election, is led by former army chief of staff Benny Gantz and two ex-generals.

As of Saturday, Netanyahu became Israel’s longest-serving prime minister, spending more than 13 years in office. He is due to face pre-trial hearings for corruption allegations weeks after the vote. He denies all charges.

When he announced his return to politics in June, Barak addressed Netanyahu directly “as your former commander” and said he “can’t continue and hold on to the reins of power”.

He said: “For your own good, and for the good of the state and everything you’ve contributed to it during your life, your time as a political leader is over.”