The High Court in London on Tuesday ruled in favour of the family of hotel worker Baha Musa, 26, who died while in the custody of British troops, but five other families had their applications for judicial reviews rejected.
All six families had claimed that they were entitled under human rights laws to independent and effective inquiries into whether troops were guilty of unlawful killing.
"Today is a historic day for human rights and the rule of law in the United Kingdom," said Phil Shiner, a lawyer for the Iraqi families.
"The High Court has ruled that in the cases of civilians killed or tortured in detention in Iraq, the European Convention on Human Rights applies to British forces," he added.
"It has ruled that there has been a violation of articles two, right to life, and three, prohibition of torture and in particular the obligation to carry out an independent investigation," he said.
A spokesman from Prime Minister Tony Blair's office said: "We will be studying the judgment."