The UK Government has challenged Iran on the treatment of Christians and other minority faith groups.
In a statement to the UN during its 34th Periodic Review on human rights, Miriam Shearman, representing the Government in Geneva, said that it was "deeply concerned" about Iran's "failure" to meet its legal obligations under international law.
Specific concerns highlighted in the statement included the arbitrary detention and mistreatment of both citizens and foreign nationals.
Ms Shearman asked Iran to demonstrate that all those being held in prison are not subjected to torture or inhumane treatment, and that those facing criminal charges can appoint their own lawyers and be given an impartial trial.
In a separate call, she said that Iran should investigate the sexual exploitation of children, including forced and early marriage.
"We remain concerned by discrimination against persons belonging to minority religious groups, particularly the Baha'i and Christians," she said.
"We remain deeply concerned by Iran's failure to uphold international legal obligations, and its arbitrary detention of citizens and dual nationals arrested on unclear charges, denied due process and subject to mistreatment."
Iran is ranked number 9 on the Open Doors list of the 50 countries in the world where Christians suffer most for their faith.
It is illegal to convert to Christianity or to preach the Gospel, and underground church pastors and members face constant risk of arbitrary arrest and imprisonment.