Ehud Barak may be Israel’s shortest-serving prime minister at one year and eight months but, at the age of 76, there are many who believe he could be the only viable challenger to Benjamin Netanyahu.

This belief seems to be shared by the prime minister’s close circle, as his supporters have been attacking Mr Barak in recent weeks with allegations of dodgy payments.

It was six years ago this week that Mr Barak — then defence minister in the Netanyahu government — announced he was leaving frontline politics.

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At the time, the prevailing wisdom in Jerusalem was that he had given up on the possibility that Mr Netanyahu would give the order a strike on Iran’s nuclear facilities, a move for which he had been one of the main supporters.

In subsequent interviews, Mr Barak bolstered that impression by hinting that the prime minister had never been serious about going ahead with the attack and was trying instead to convince the United States to strike Iran.

Since then, Mr Barak has focused on private business but, after spending four years in government as Mr Netanyahu’s closest ally, he has emerged as one of his fiercest critics in interviews and on social media.

Every two or three days, he tweets on the prime minister’s failings. A tweet from November 13, in the wake of the latest round of warfare in Gaza, was a prime example: “Bankruptcy of the prime minister. The king is naked. There was never such a gap between hollow talk and operational impotency.”

פשיטת רגל של ראש הממשלה. המלך עירום. מעולם לא היה פער כזה בין דיבורים חלולים לאימפוטנטיות ביצועית. במילים- נמוטט את החמאס, 48 שעות לחיסול הנייה. במעשים- הפקרת ה'עוטף' וכניעה תחת אש לחמאס. עאלק ׳שומר ישראל׳. סנוואר משפיל את ביבי. הוא המחליט מתי היתה אש. מתי תיפסק, ומתי תתחדש. עלוב pic.twitter.com/jGlKxhsR5Y

— אהוד ברק (@barak_ehud) November 13, 2018

Ehud Barak’s electoral record is far from stellar.

As Labour leader, he beat Mr Netanyahu in the 1999 election but went on to lose to Ariel Sharon in 2001 and again to Mr Netanyahu in 2009.

But with the centre-left in Israel currently split between Zionist Union (the joint list of Labour and Tzipi Livni’s Hatnuah) and Yesh Atid, and both parties polling currently in the mid-teens, there are many within Labour who privately wonder whether they would not be better off with him replacing the present party leader Avi Gabbay.

Ms Livni, who in the past was estranged from Mr Barak, has taken to calling for a “bloc” of centre-left parties and holding an open primary for its leader — a proposal seen by many as preparing the ground for a return of the last Labour prime minister.

Mr Barak himself in his interviews denies he has any concrete plans. In July he said: “I’m not in mind to make a comeback. I’ve already been in all the positions.”

He describes himself as “a concerned citizen” — but neither does he rule out the possibility.

It looks like Mr Netanyahu is also concerned.

In recent weeks, journalists close to him have unearthed payments Mr Barak received in 2004-06, while he was not in public service, from the American Wexner Foundation, which runs high-level programmes for Jewish leaders and senior Israeli civil servants.

The foundation paid Mr Barak a total of 2.3 million shekels (£480,000) for undisclosed “research”.

The former prime minister did not deny receiving the payments, but insisted he paid tax on them.

Mr Barak also hit back, claiming he was the victim of a smear campaign by “proxies” of Mr Netanyahu, who he described as “terrified of an imminent indictment and the toppling of his house of cards”.

It seems that rather than inhibit him from making a comeback, this may have invigorated him.