An investigation by the All Party Parliamentary Group (APPG) on International Freedom of Religion or Belief found that Christians continue to be arrested and interrogated because of their faith.
After hearing from 13 witnesses and anti-persecution charities the group published its report.
Christians "continue to be treated harshly, with some facing severe physical and psychological torture during periods of detention," it said.
"The judiciary continues to construe legitimate Christian activities (such as meeting in private homes for prayer meeting and bible studies, or being in contact with Christians outside of Iran) as political activities that threaten the national security of Iran.
"Therefore Christians continue to be issued long prison sentences and/or corporal punishment.
"Churches continue to be pressured into ceasing all services or activities in the national language of Persian (Farsi), or are closed down.
"Property belonging to Christians has continued to be seized, and Christians continue to face discrimination in the workplace and in educational institutions."
The APPG said it had tried arrange a trip to Tehran for talks with the Iranian authorities on matters relating to freedom of religion or belief but had been rejected.
Iran's government had "no intention" of allowing the politicians to visit, the report said.
After the election of President Hassan Rouhani in August 2013 there had been a hope that life for Christians might improve under new leadership, but the report said this had not happened.
Ajay Sharma from the Foreign and Commonwealth Office told the APPG: "There has been no substantive change in Iran's human rights record since the election of President Rouhani; in fact by some indicators you could argue that things have gotten worse."
Iranian Christian Elham said: "I had a slight hope that with Rouhani there would be more freedom... So I gave him a vote. But no laws have been changed. There is no increase in freedom for Christians."
Another Christian Mostafa Bordbar said: "During Ahmadinejad's government, they closed many formal churches in Iran, and we had to go to home churches. And they told us- especially Ayatollah Khamenei said very officially - that home churches are against the law, and you should not go to home churches, just the official churches. And they closed the official churches - so what should we do?"
Chair of the APPG Baroness Berridge told Premier: "There's information that the Church is actually growing in Iran.
"People are questioning Islam because of its link to the regime and people are not happy with the regime.
"They question the faith that is behind [the government] and so there are people wanting to find out more about other faiths, particularly Christianity.
"So, yes, we think that the Church in Iran is growing, so there is hope there."