A reservist colonel in the Israeli army has resigned his commission in protest at his army's "immoral conduct" in the occupied territories. In a searing open letter to the army's chief of staff, Lt-Col Eitan Ronel, a veteran of 1973's Yom Kippur War, the invasion of Lebanon, and the first Palestinian Intifada, returned his officer's commission.

His resignation came even as five teenage conscripts were yesterday sentenced to a year in prison each for refusing to serve in the Israeli army "as long as it acts as an army of occupation". Scores of reservists have refused to report for duty for similar reasons, and many of them have been sentenced to prison terms, but Lt-Col Ronel, 51, is believed to be the first Israeli officer to resign his commission in protest.

In his letter to the chief of staff, Lt-Col Ronel, who was released from active reserve duty two years ago, wrote that for him, the final straw had been when Israeli soldiers opened fire last week on unarmed protestors demonstrating against the "separation fence" Israel is building in the West Bank. Among the injured was an Israeli civilian, Gil Naamati, whose case provoked a storm of controversy in Israel.

"A country in which the army disperses demonstrations of its citizens with live gunfire is not a democratic country," Lt-Col Ronel wrote. "An army that educates its soldiers that such a crime is conceivable has lost all its borders.

"I saw this deterioration, stage after stage: the blind eye that was turned to the abuse of detainees in violation of the army's orders; the blind eye that was turned to soldiers' gunfire on unarmed Palestinian civilians; the blind eye that was turned to the settlers' unlawful behaviour towards Palestinian civilians; the oppression of the population; the roadblocks; the curfew; the closure; the blind eye the army turned towards humiliation and abuse; the searches and arrests; the use of live fire against children and unarmed people."

Lt.-Col. Ronel continued: "Stage by stage, the value of human life has diminished. Step after step, the values on which we were raised-the purity of arms, the value of human life, the dignity of human beings as being created in the image of God-have become a scornful travesty. And now we have reached the next stage: soldiers shoot at Israeli civilians in a demonstration, in keeping with the regulations for opening fire...This is an educational, ethical and moral failure.

"This is your failure," he told the chief of staff, "[the failure] of the army commanders. If I had any faith in you, I would say to you-clean out the stables, take responsibility, resign. My faith in you is gone. You have failed. You and your predecessors have corrupted my army, our army. I do not want to be a part of such an army. You gave me the ranks, to you I return them."

Lt-Col Ronel's decision will resonate deeply in a country where military service is at the heart of society, and achieving senior military rank is considered a great mark of distinction. It comes after a group of reservists pilots, including a highly decorated war hero, refused to fly any further missions to assassinate Palestinian militants over the occupied territories, because of the civilian casualties involved - which provoked a furore.

Though directed at the chief of staff, Gen Moshe Ya'alon, Lt-Col Ronel's criticisms will also be seen as criticisms of Ariel Sharon, under whose premiership most of the abuses he refers to have taken place. Gen Ya'alon himself recently spoke out against the government's military tactics in the occupied territories, saying they were counter-productive and fostered Palestinian hatred of Israel.

The Israeli army spokesman's office said the army had not received Lt-Col Ronel's resignation letter yet, but that military officials had seen it on the internet. "At issue is an individual who is using his rank and position to voice a political position and is trying to involve the army in politics," a spokesman said.