Blair to defend record over Iraq
Tony Blair is set to mount a spirited defence when he is questioned in public for the first time about his decision to take the UK to war against Iraq.
He will be questioned at the Iraq war inquiry for six hours on the build-up to the 2003 invasion and its aftermath.
Controversial government dossiers justifying action will be discussed.
BBC political editor Nick Robinson said the ex-PM was expected to say Saddam Hussein had the "capacity and intent" to build weapons of mass destruction.
He added: I'm told that Tony Blair will claim that the fall of Saddam has improved and saved the lives of many Iraqis.
"He'll argue that despite the terrible bloodshed since, it has been worth it for Iraq and the world as a whole.
Demonstrators outside the inquiry venue will demand to know from Mr Blair where the weapons of mass destruction were that Saddam was supposed to have, the BBC's Nick Robinson went on.
"His answer, I'm told, is this. The weapons inspectors found Saddam had both the capacity and intent to build them at speed."
The session gets under way at 0930 GMT.
Families of some of the 179 British soldiers killed in Iraq are expected to take part in an anti-war demonstration outside the inquiry building in central London.
Mr Blair's biographer, Anthony Seldon, said: "It's a pivotal day for him, for the British public and for Britain's moral authority in the world."
Prime Minister Gordon Brown, who will shortly face a grilling by the inquiry himself, said he was not concerned about Mr Blair's appearance before it.
He told Sky News: "Tony Blair is able to set out the case, to show the decisions he made, and to do so in the most professional and eloquent way, and I believe that he will be able to answer all the questions that the inquiry puts to him."
Liberal Democrat leader Nick Clegg, who opposed the war, writing in an article for Friday's Daily Telegraph, said Mr Blair's appearance would be "a pivotal moment in answering a question millions of British people are still asking themselves: Why did we participate in an illegal invasion of another country?"
He said the invasion of Iraq was an example of "subservience by default to the White House" which raised wider questions about the "special relationship" between Britain and America.
The Chilcot inquiry is also likely to ask Mr Blair at what stage he promised US President George W Bush Britain would support military action against Iraq.
Some witnesses have said assurances were given in 2002 - although the then Attorney General Lord Goldsmith has told the inquiry he had warned Mr Blair that using force for regime change would be illegal.
Lord Goldsmith, whose legal opinion the British government relied on to justify their involvement in the war, said he originally believed the United Nations had to approve the use of force and only changed his mind a month before the invasion.
Mr Blair has already pre-empted one potential line of questioning by saying in a recent BBC interview that he would have backed the invasion even if he had known beforehand that Iraq had no weapons of mass destruction.
A former head of the civil service, Lord Turnbull, told the inquiry they should question Mr Blair about this because throughout the period leading up to war, Mr Blair had been "unambiguous" that disarming Saddam was his primary objective.
In his appearance, Mr Blair's former director of communications Alastair Campbell sought to address some of the questions around the key September 2002 dossier on the alleged threat posed by Saddam published in the run up to the war.
The dossier included a foreword by Mr Blair in which he wrote that he believed the intelligence had established "beyond doubt" that Saddam Hussein had continued to produce chemical and biological weapons.
Sir John Scarlett, chairman of the Joint Intelligence Committee, told the inquiry in his evidence that the foreword was "overtly political" and "quite separate" from the rest of the dossier.
Mr Campbell, who drafted the first version of the foreword - ultimately approved by Mr Blair - said no-one in intelligence challenged this statement which, he added, never suggested Saddam Hussein "was able to do something terrible to the British mainland".
On the claim that Saddam's weapons could be deployed within 45 minutes of the order being given, which was retracted after the war, Mr Campbell said it could have been clearer but had only been given "iconic" status by the press.
He said Mr Blair's policy had only ever been one of seeking Iraq's disarmament, by diplomacy if possible, by force if necessary, telling the panel: "He really believed in it."
But he also revealed Mr Blair had written to President Bush saying that if there was to be war to disarm Saddam, "Britain will be there." The letters have not so far been published.
This will be the third time Mr Blair has given evidence during an inquiry into the Iraq war.
He previously gave evidence to the Hutton inquiry, the Butler review and the Intelligence and Security Committee investigation - although the latter two took place behind closed doors.
Questions at the Hutton inquiry were restricted to events surrounding the death of government weapons scientist Dr David Kelly, rather than the political decisions behind the war.