The International Federation of Iraqi Refugees (Ifir) has raised concern over the welfare of the asylum seekers once they arrive in Iraq, which has seen a continuation of deadly suicide bombings in recent months.
The Ifir said 39 people had been deported on a specially chartered Air Italy flight that left London's Stansted airport early on Thursday.
The group said people on the flight were told it was going to Baghdad, making it the first deportation flight into south or central Iraq from the UK.
Previously, Iraqi deportees from Britain have been flown into the Kurdish region of northern Iraq, which has not seen the level of violence experienced in other parts of the country.
Speaking to Al Jazeera from London, Richard Whittel, from the Coalition to Stop Deportations to Iraq, said: "People were woken up at the dead of night last night and put on coaches from detention centres and then taken to Stansted airport.
"We think they've been put on a plane operated by Air Italy and thought to have been taken to Baghdad.
"The Home Office [UK interior ministry] argument would be that Iraq is now safe to send people back to, people aren't at risk, and people's asylum applications have failed.
"We're used to the macabre immigration policies of the UK government, and indeed other European governments, but this is an especially malicious step sending people to Baghdad."
A spokesperson for the UK border agency, which is alleged to have issued letters to the refugees about deportation, said on Wednesday they could not comment on the "timing or detail of removal operations".
But it added: "Anyone who is in the UK illegally is expected to leave and the UK Border Agency will not hesitate to enforce their removal where necessary.
Hussein, one of the Iraqis said to be facing deportation, told the Ifir he could be killed if he returned to his homeland.
"I'm from Baghdad. I'm a Sunni - I have a big problem with a Shia party. They've used their influence to imprison my brother and they're looking for me.
"If I'm deported I know 100 per cent I'm going to be killed."
According to Britain's interior ministry, 632 people were deported to northern Iraq between 2005 and 2008.
Source: Al Jazeera (English)