The campaign group said 30,000 detainees – many of whom were transferred from US custody – are at risk of torture and mistreatment, while being denied proper judicial process.
The wife of Ramze Shihab Ahmed, a dual Iraqi-UK national who claims to have been tortured in Iraq, today called on the Government to increase efforts to secure his release.
Rabiha al-Qassab said her husband has suffered electric shocks to his genitals and had plastic bags placed over his head during his ordeal at prisons in Baghdad.
Mr Ahmed, 68, was arrested in December after travelling to Iraq in an attempt to free his detained son, Omar.
His whereabouts were unknown until March when he was able to make a short phone call to his wife in London.
Rabiha al-Qassab said: "What my husband has suffered at the hands of his interrogators is inhumane and sickening.
"I'm desperately worried about him. He already had health problems before all this and was very brave to return to Iraq on behalf of his son Omar in the first place."
She called on the UK Government to increase its efforts to get her husband released, or at least push for a fair trial.
In a new report out today, Amnesty estimates 30,000 detainees are being held without trial in Iraq.
About 10,000 of these were recently transferred from US custody following the end of American combat operations in the country.
The report claims several detainees have died while in prison, apparently as the result of torture or ill-treatment.
One man, Riyadh Mohammad Saleh al-Uqaibi, died in February of internal bleeding, having been beaten by guards so hard that his ribs were broken and his liver damaged.
Malcolm Smart, Amnesty International's Middle East and North Africa director, said: "The Iraqi authorities have signally failed to take effective action to stop torture and punish the perpetrators despite overwhelming evidence to its use."