Israeli cabinet minister Natan Sharansky has submitted his resignation in protest at Prime Minister Ariel Sharon's plans to withdraw from Gaza.
Sharansky has been a vehement opponent of the pullout
He told Israel Army radio the plan was "a tragic mistake that exacts a high price and also encourages terror".
The pullout of all 8,000 Gaza settlers and four West Bank settlements is due to happen in July or August.
Mr Sharansky's resignation is unlikely to sway the prime minister, says the BBC's Lucy Williamson in Jerusalem.
The former Soviet dissident leads immigrants' rights party Yisrael B'Aliyah and serves as minister of diaspora affairs in Ariel Sharon's cabinet.
Several Israeli cabinet ministers have either been sacked or have resigned over the Gaza plan.
Mr Sharansky said he was resigning because the Gaza withdrawal plan was not conditional on democratic reform of the Palestinian Authority.
"Waiving this condition will weaken the chances of building a free Palestinian society and support terror," he wrote in his letter of resignation.
In an interview with Israel Army radio, quoted by the Associated Press news agency, he said: "Since the only justification for the existence of the government in its current composition is to carry out the disengagement plan, I don't think i can be part of it."
Many Jewish settlers are fiercely opposed to the Gaza plan
Despite splits in his own Likud Party, which forced him last year to seek a new coalition with the opposition, Mr Sharon seems determined to push ahead with disengagement.
Jewish settlers have held a number of protests to topple Mr Sharon's government.
But opponents of the disengagement plan have exhausted all legal and political avenues and their protest looks increasingly beleaguered, says our correspondent.
Last month, Israel's defence minister recommended a three-week delay in pulling out of Gaza.
The proposal is expected to be endorsed by Mr Sharon - pushing the move back from late July to mid-August.
The Israeli government is concerned that the original date clashes with a period of mourning for the destruction of two biblical-era temples.
There has been speculation that the delay is designed to buy time because the authorities are not ready.
Israel has occupied Gaza and the West Bank since the 1967 Middle East war.