Lawyers demanding a public inquiry into allegations of beatings by prison officers at Wormwood Scrubs in the 1990s have started legal action.
Three prison officers were jailed
Home Secretary David Blunkett had been warned by the inmates' legal team he had until 1600 BST on Friday to order an inquiry, or face legal action.
They say the prison service was aware of brutality at the west London jail during the 1990s, but failed to act.
One of the solicitors, Daniel Machover, said legal action will begin next month when the High Court will be asked to grant an order requiring the home secretary to hold an inquiry.
Accusations of mistreatment at the jail led to the biggest ever criminal investigation at a British prison, with 27 officers suspended from duty and prosecuted - three of whom were convicted.
Following the allegations and investigation into the conduct of 100 officers, prisoners also won a number of civil claims.
But lawyers for the group say a culture of violence had taken hold at the prison, and they want to know whether there was a cover-up of what allegedly went on.
The chief inspector of prisons and the Prison Officers Association both support the call for a public inquiry.
If Mr Blunkett does not respond to the request by 1600 BST, the lawyers will ask the courts to compel him to set one up.
Last September, the Chief Inspector of Prisons warned that Wormwood Scrubs jail was still riddled with serious problems.
Anne Owers said the jail had "stalled", or was "sliding backwards" in areas that had begun to improve.
Criticisms in her report at that time focused on a health care centre in an "appalling state", almost half of inmates questioned saying they sometimes felt unsafe in the prison, and an "ineffective" anti-bullying strategy.
Her report said that although the 1,000-inmate prison was rid of rogue officers, it was struggling to move forward and make progress.
But the Director General of the Prison Service, Martin Narey, said some of the criticisms were unfair.
He said: "The unacceptable treatment of prisoners and the use of violence against them has been eradicated at Wormwood Scrubs."