Prime Minister Benjamin Netanyahu’s two biggest challengers have joined forces for the April elections, piling more pressure on the embattled premier who is standing for re-election under the shadow of possible graft charges.

Lt General Benny Gantz, 59, the former head of Israel’s army, and Yair Lapid, a former journalist and chair of centrist party Yesh Atid, announced on Thursday they would run on a joint ticket, and if they win the 9 April polls, would agree to a rotation for the prime minister’s spot.

They also announced they were enlisting the support of Lt General Gabi Ashkenazi, Gantz’s predecessor as chief of staff, further bolstering the security credentials of the alliance which already includes Moshe “Bogie” Ya’alon, also an ex-army chief.

“Motivated by national responsibility, Benny Gantz, Yair Lapid, and Bogie Ya’alon have decided to establish a joint list that will constitute the new Israeli ruling party,” a spokesperson for the campaign told The Independent on Thursday.

“The new ruling party will bring forth a cadre of security and social leaders to ensure Israel’s security and to reconnect its people and heal the divide within Israeli society,” she added.

Details of the alliance are due to be announced on Thursday evening during a joint press conference in Tel Aviv.

A campaign spokesperson said that the two leaders agreed that should they win Lt Gen Gantz would hold office for the first two and half years and Mt Lapid would assume the position afterwards.

Sources close to the pair said they held “marathon talks” through the night to hammer out a deal.

The news dealt a damning blow to Mr Netanyahu, who is hoping to be serve a fifth term, which would make him the longest serving prime minister in Israel’s history, surpassing even the country’s founding father David Ben-Gurion.

However his popularity has tanked in recent months as the shadow of possible indictment loams over him.

His Likud party, which heads up what is considered to be Israel’s most right-wing government in history, were quick to blast the new union as “left wing” and pro-Arab.

“The choice is clear: It’s either a left-wing government headed by Lapid and Gantz and supported by a bloc of Arab parties, or a right-wing government headed by Netanyahu,” the party said in a statement.

Yuli Edelstein, speaker of the Israeli parliament and a top member of Likud, meanwhile said the centrist merger made it more imperative for the right-wing to present a “united and strong” front.

“Any alternative will set Israel back by decades in terms of its economy and security,” he wrote on Twitter.

Benny Gantz (right) a former head of the IDF (centre) and head of Israel resilience party Moshe Ya’alon (Getty) (Photo by Amir Levy/Getty Images)

But it is unclear if the scaremongering is working.

The alliance of three powerful former army chiefs, and the long-standing opposition figure MR Lapid, effectively wrestles back Mr Netanyahu’s main selling points: his security credentials and political veterancy.

According to the latest polls published this week by Israel’s Channel 12 news, Mr Netanyahu’s right-wing Likud party would secure 30 of the 120-seat Knesset, putting him ahead of the other parties and allowing him to lead a ruling coalition.

However, the same poll, which was released before Thursday’s announcement, predicted that a Gantz-Lapid merger would see them better Likud’s performance with 32 of the seats.

Israeli left-wing paper Haaretz described the merger as a “political bombshell” for Mr Netanyahu.

Yair Lapid, chairman of Israel’s Yesh Atid party, presents his electoral list this week (AFP/Getty) (JACK GUEZ/AFP/Getty Images)

The Times of Israel meanwhile called the alliance a “game changer” and said it meant that “for the first time in years, the prime minister has a fight on his hands”.

Affecting the 9 April polls is an expected decision by Israel’s attorney general whether to push ahead with indicting Mr Netanyahu on three cases that are currently levelled against him, dubbed 4000, 2000 and 1000.

In case 4000, police have alleged that Mr Netanyahu granted regulatory favours to Israel’s leading telecommunications company, Bezeq, in return for more positive coverage on Walla, a news website belonging to the firm’s owner.

Case 2000 focuses on suspicions Mr Netanyahu negotiated a deal with the Yedioth Ahronoth newspaper for better coverage in return for promises to limit the circulation of a rival. In the third investigation, case 1000, police argue that he received expensive gifts from wealthy friends.

Mr Netanyahu has vehemently denied the charges and vowed to remain in office even if formally charged.

Further denting his popularity this week was an embarrassing diplomatic spat with Poland which saw the collapse of Mr Netanyahu’s long-awaited Visegrad summit that he was due to host in Jerusalem.

The right-wing Israeli leader had hoped to shore up support abroad by hosting four central-European leaders, from Poland, Hungary, the Czech Republic and Slovakia. However, Poland pulled out over statements made by Mr Netanyahu and his acting foreign minister that Poles collaborated with the Nazis in the Holocaust.

Both Lt Gen Gantz and Mr Lapid, a former journalist turned politician, have played up these issues in their electoral campaigns, portraying Mr Netanyahu as haughty with power and criminally corrupt.

On Tuesday Gantz unleashed a stinging rebuke of Mr Netanyahu chiding him for his long years studying and working in the United States and accusing him of becoming “addicted to the pleasures of power, corruption and hedonism”.

The premier struck back immediately, highlighting his own military experience. “Benny Gantz, be ashamed of yourself,” the prime minister said in a video clip, saying he had risked his life “time and time again for our country”.