Amnesty International is calling for Iraqi lawmakers to drop proposed changes to the Personal Status Law that could allow girls as young as nine and boys under 15 to be married.
The rights groups are speaking out before an imminent parliamentary vote on the changes.
The Personal Status Law governs matters related to marriage, divorce, child custody and inheritance for Muslims in Iraq and applies to all Iraqis regardless of their religion.
However, the new changes would grant Islamic religious councils to develop their rulings on personal status matters, which the rights group say effectively threatens "women's and girls' rights and their equality before the law".
"Not only does child marriage deprive girls of their education, but married girls are more vulnerable to sexual and physical abuse, and health risks related to early pregnancy," Amnesty said.
"It is alarming that these amendments to the Personal Status Law are being pushed so vehemently when completely different urgent legal reforms are needed to protect Iraqi women and girls' rights."
The current legal age for marriage is 18 for boys and girls, but the new amendment will allow couples getting married to choose whether the conditions of the law or Islamic teachings would apply – undermining legal equality.
Human Rights Watch (HRW) says the changes would "separate legal regimes for different sects and further entrenching sectarianism in Iraq".
"In practice, passing this amendment would mean that one girl in Iraq may be protected from child marriage, while her classmate is forced into it," HRW said.
The amendments will also "open the door" to unregistered marriages, which Amnesty says are often used to circumvent child marriage laws, remove penalties for adult men who enter these marriages and clerics who conduct them and remove protections for divorced women.
The bill's first reading took place on 4 August. However, on 3 September, Iraq's parliament tried to hold a second reading, but opposing MPs waged a boycott campaign that resulted in few members showing up.
However, the second reading took place on 16 September, much to the concern of female MPs, and the Iraqi Federal Supreme Court ruled the changes aligned with the nation's constitution the next day.
Similar amendments were proposed in 2014 and 2017 but failed to pass after a national outcry.