The detainees were found when American troops surrounded and took control of an Interior Ministry building in the Jadriya neighbourhood of Baghdad on Sunday night.

The raid was carried out after reports that detainees were being illegally held and tortured there.

When US forces arrived, officials said that only 40 detainees were being held. But as troops moved through the building, opening door after door, they found scores of prisoners, many in very poor health. The Americans had apparently been tipped off to the prison’s existence by relatives of those being detained there.

It appears to be the country’s new Abu Ghraib, the notorious Baghdad prison where Iraqis were pictured being humiliated by American soldiers.

The discovery came as Republican leaders in the Senate called on President Bush yesterday to set out a strategy allowing for a withdrawal of 160,000 US troops from Iraq. It is seen as a clear sign that the increasingly unpopular war is unnerving the President’s own party.

The Senate’s Republican leadership, usually loyal to the White House, demanded that 2006 be a “significant transition” year in which Iraqi forces took the lead in securing their country, so that US troops could begin a phased withdrawal. The proposal by Bill Frist, the Republican Senate Leader, and John Warner, the veteran Virginian Republican and chairman of the powerful Armed Services Committee, was passed by 79 votes to 19.

Official announcement of the secret jail’s discovery came a day after a damning United Nations report into the brutal conditions and lack of access to legal counsel in Iraq’s overcrowded prisons.

The UN Assistance Mission in Iraq expressed concern about the large number of detainees being held in the country’s prisons and suggested that Iraqi police and special forces had abused the human rights of suspects during security sweeps.

But the discovery of an apparently illegal detention centre has raised even more questions over the behaviour of the security forces in Iraq, which are being primed to take over duties from a withdrawing coalition force.

It also revived memories of how the security forces behaved under Saddam Hussein, who routinely had people arrested and tortured at secret prisons and detention centres around the city, many of which were not discovered until after his regime had fallen.

Ibrahim al-Jaafari, the Prime Minister, said that there would be an investigation into allegations that Interior Ministry officials had tortured detainees held in the basement prison in connection with the mostly Sunni insurgency.

“I was informed that there were 173 detainees held at an Interior Ministry prison and they appear to be malnourished,” Mr al-Jaafari said. “There is also some talk that they were subjected to some kind of torture.”

He said that the detainees had been moved to another location and were receiving medical care. American military officials refused last night to comment on their role in bringing the secret prison to light, referring all questions to the Interior Ministry.

Major General Hussein Kamal, the Interior Ministry’s under-secretary for security, confirmed that the raid on the building had taken place but said that all those being held in the facility were “terrorists”.

However, during a chance street encounter, Brigadier General Karl Horst, who commanded the troops that carried out the raid, said that US forces now planned to raid all known detention facilities in the capital. “We’re going to hit every single one of them,” he told the Los Angeles Times. The raid was among the first known instances in which American forces in Baghdad have stepped in to protect prisoners being held in Iraqi detention.

The discovery will do nothing to calm relations between the country’s warring ethnic groups. Most of the prisoners discovered in the jail were Sunni Arabs, members of the minority ethnic group dominant under Saddam but who make up the backbone of the insurgency fighting the military occupation and the US-backed Government.

The Interior Ministry, controlled by Shias, the oppressed majority under Saddam, has been accused of using its security forces to detain, torture and kill hundreds of Sunnis because of their religious affiliation.

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