On Wednesday, Hanegbi, a minister without portfolio, became the sixth minister to follow Sharon out of Likud as part of the continuing turmoil in Israeli politics in the run-up to a March general election.
"My heart was telling me to stay in Likud but my head told me otherwise and so therefore I am today announcing my resignation from Likud after representing the party for the last 17 years in the Knesset," he told a press conference.
"Prime minister, I am with you," Hanegbi said. "Israel can place its trust in Ariel Sharon, who has always fought mercilessly against terrorism and has never compromised on Israel's security."
Hanegbi said he would stand down as an MP immediately but would remain in the cabinet.
The move came amid reports that police are to recommend Hanegbi's prosecution on charges of cronyism during his time as environment minister.
While the other five Likud ministers to have joined Sharon were seen as close allies of the premier, Hanegbi was regarded as an opponent of his controversial decision to pull troops and settlers out of the Gaza Strip.
As interim head of the Likud central committee, the party's main governing body, Hanegbi was to have played a key role in organising the leadership election to replace Sharon due on 19 December.
Sharon's Gaza withdrawal was
apparently opposed by Hanegbi
The defection will be seen as another major blow to Likud, which won 38 out of the 120 seats in the Knesset at the last election but is now set to win only a dozen, according to an opinion poll in Wednesday's Haaretz newspaper.
The survey found Kadima was on course to emerge as the largest party with 39 seats after the 28 March election, comfortably ahead of the centre-left Labour with 22.
The findings demonstrate how Kadima has been able to shatter the mould of Israeli politics, which had been essentially a two-party system, since its formation on 21 November.
Break from party
Apart from the Likud defectors, Sharon has also won the endorsement of the former Labour leader Shimon Peres.
Sharon decided to break from the party he was instrumental in founding 32 years ago, fed up with battling hardliners who refused to forgive him for pulling out of Gaza after a 38-year occupation.
In a speech on Wednesday by the tomb of the Jewish state's first prime minister, David Ben Gurion, the current premier indicated that Israel must be ready for more pullouts from the West Bank.
The exit of Hanegbi (R) has dealt
a blow to the conservative Likud
"In order to achieve peace, we are ready to give up some of what is rightfully ours," Sharon said during a ceremony in the southern Negev desert.
Former premier Benjamin Netanyahu, who is favourite to regain the Likud leadership, insisted security and military strength would bring peace to Israel as he took a swipe at his arch rival Sharon.
"Our peace is dependent on our strength ... and not on endless (territorial) retreats," he said in his first press conference since Sharon's dramatic split from Likud.
Before news emerged of Hanegbi's switch, there were widespread reports that police were about to submit a file on the minister to the state prosecutor which contained enough evidence to justify pursuing a case through the courts.
Police opened an investigation in August last year after a report by state comptroller Eliezer Goldberg found Hanegbi had "trampled on the law" by appointing dozens of political supporters to posts while environment minister.
He subsequently stepped down as public security minister but remained in cabinet.
Asked about the reports of an imminent prosecution, Hanegbi dismissed them as "newspaper speculation".