Bush's policy on terror suspect detainees has been criticised

What is the significance of this announcement?

It had long been thought that the US was holding at least some suspected terrorists in secret CIA prisons, not least since the whereabouts of some prominent figures such as Khalid Sheikh Mohammed were unknown for years.

Mr Bush's announcement officially acknowledges that the secret CIA detention programme exists and that it uses "an alternative set of procedures" to elicit information from suspects trained to resist interrogation.

He said he could not reveal the what the methods were because "it would help the terrorists learn how to resist questioning".

He added that the interrogation techniques did not constitute torture, which he said he had not authorised and would never authorise.

And he detailed what he said were a number of plots against the United States and its allies that had been prevented because of information gleaned from CIA interrogations.

Why did President Bush acknowledge the existence of the CIA programme now?

He himself said he was doing so because the questioning of the 14 suspects in CIA custody was complete and the men were now being transferred to US military custody at Guantanamo Bay, Cuba.

The administration had been planning to try at least some of the Guantanamo Bay detainees by military commission, but the US Supreme Court ruled earlier this year that there was no legal framework for doing that.

Mr Bush has now proposed legislation that would allow the military trials to go ahead, but the clock is ticking - Congress is sitting for only a few more weeks before they recess to campaign for mid-term elections, scheduled for 7 November.

The president also had one eye on the fifth anniversary of 9/11, saying the families of those killed in the attacks had waited long enough for justice. He said the US wanted to bring Khalid Sheikh Mohammed and the other suspects to trial.

Why have the secret prisons been controversial?

Word of the secret prisons was leaked to the Washington Post newspaper, which reported that the US had set up "black sites" abroad for the interrogation of high-value suspects.

The European Parliament called for an investigation in light of the suspicion that some of the detention centres were in Europe, while the US Senate demanded information from the country's director of national intelligence.

Human rights groups also criticised the alleged secret detention programme as being beyond the rule of law and not affording sufficient protection to detainees.

The US allowed the International Committee of the Red Cross to have access to Guantanamo Bay, but not to the secret CIA prisons.

What happens next?

The 14 suspects mentioned in Mr Bush's 6 September speech - including Khalid Sheikh Mohammed, Ramzi Binalshibh and Abu Zubaydah - have already been transferred to Guantanamo Bay.

Mr Bush hopes to get legislation quickly through Congress to authorise military tribunals of the detainees, but it is not clear whether or not that is realistic.

Every member of the House of Representatives and one in three senators is running for re-election in November.

No candidate will want to be seen as soft on defence, but Mr Bush is a fairly unpopular figure in the US at the moment, so it is also not necessarily to a candidate's advantage to be too closely linked to him.

And even members of Mr Bush's own Republican party - such as Senator John McCain - are already expressing doubts about details of the president's proposals, such as barring defendants from seeing all the evidence against them.

Meanwhile, the secret CIA detention programme will continue to exist, even with no-one in custody, the president said, because having it in the future "for questioning terrorists will continue to be crucial to getting life-saving information".