One of Tony Blair's closest advisers will be grilled by the Iraq Inquiry today as it starts a week of evidence sessions dominated by key political figures.
Jonathan Powell, who was the former UK prime minister's chief of staff throughout his time in Number 10, is due before John Chilcot's panel this afternoon.
The former Blair aide is likely to face tough questions over the part he played in producing the British government's dossier on Iraq's supposed weapons of mass destruction.
He was also present at a key meeting at US President George Bush's Crawford ranch where the inquiry has been told Mr Blair committed to military action 11 months before the invasion.
He will also be expected to provide fresh light on secret memos he saw between the two leaders in the run up to the war in which Mr Blair assured Mr Bush Britain would "be there" if it came to war.
Details of those highly-sensitive notes were recounted to the inquiry last week by former Downing Street spin chief Alastair Campbell.
In a charged session, Mr Campbell also denied trying to "beef up" the 2002 dossier and robustly defended the government's policy, suggesting the country should be "proud" of its role.
Mr Powell was previously revealed to have suggested changes to the assessment of Saddam Hussein's willingness to deploy such weapons, telling security chiefs the wording was "a bit of a problem".
Mr Blair used the dossier, including the claim that WMD could be deployed in 45 minutes, to back the case for military action despite experts deeming the intelligence "patchy".
Mr Campbell last week also rejected the earlier evidence of Sir Christopher Meyer - then Britain's ambassador to the US - that Crawford marked a fundamental shift in Mr Blair's position.
Tomorrow, the inquiry's focus will shift to the defence secretary at the time of the invasion Geoff Hoon, who is likely to be questioned about failures to adequately equip British troops.
There have been suggestions that Mr Hoon, who jointly led a botched coup against Gordon Brown this month, could level criticisms at the now British prime minister over his then role as Chancellor.
On Thursday UK Justice Secretary Jack Straw, who as British Foreign Secretary was the second most involved Cabinet minister after Mr Blair, will appear before the panel.
A memo from Mr Straw, written 10 days before the Crawford meeting and expressing apparent doubts over the legality and justification of the war, is expected to be among key pieces of evidence.
Mr Campbell faced calls for him to return to face the panel yesterday after he issued a "clarification" of his previous evidence amid fears the official record of his answer to one key question could have given "the wrong impression".
"Reading the bald words on the page gives the wrong impression of what I was saying in response to what I thought I was being asked in a question which contained a number of points in parentheses, and I would be grateful of the opportunity to make that clear to the committee," he said in a written memo.
Former Liberal Democrat leader Menzies Campbell said the note had done nothing to clear up the issue and called for Mr Campbell to be recalled.
"These issues go right to the heart of the deliberations and the responsibilities of the committee, this is why clarification is key."
Mr Blair will give a full day of evidence to the inquiry into the war at some point in the fortnight between January 25 and February 5.
A weekend poll found almost a quarter (23%) of UK voters believed Mr Blair deliberately misled MPs over the Iraq war and should face war crimes charges.
Mr Straw's note, dated March 25 2002 and marked "Secret and Personal", suggests that Mr Blair was already considering the possibility of military action in Iraq a year ahead of the invasion.
It also shows that the then British Foreign Secretary warned him that war to topple Saddam Hussein would be unpopular within the Parliamentary Labour Party and difficult to justify legally.
"The rewards from your visit to Crawford will be few," wrote Mr Straw. "The risks are high, both for you and for the government.
"I judge that there is at present no majority inside the PLP for any military action against Iraq."
Labour MPs were aware that Saddam's regime was "bad", but ministers had "a long way to go" to convince them over the level of threat from Iraq, the legal justification for war and the prospect that the result of military action would be a more compliant and law-abiding government in Baghdad.
"The Iraqi regime plainly poses a most serious threat to its neighbours, and therefore to international security," said Mr Straw.
"However, in the documents so far presented it has been hard to glean whether the threat from Iraq is so significantly different from that of Iran and North Korea as to justify military action."
Readiness to consider military action at that point appeared to be based not on any increased threat from Iraq but on the reduced tolerance of the world community in the wake of the September 11 attacks on the US, he suggested.
Mr Straw warned of "two potential elephant traps" over the legality of military action - revolving around the vexed questions of regime change and further United Nations resolutions.
"Regime change per se is no justification for military action; it could form part of the method of any strategy, but not a goal," warned Mr Straw.
Removing Saddam's government could be presented as an essential part of the elimination of Iraq's weapons of mass destruction, but could not itself be the goal of the invasion.
Mr Straw said that the US was likely to oppose any requirement for a fresh UN Security Council resolution before taking military action, but "the weight of legal advice here is that a fresh mandate may well be required".
A clear UN mandate would "transform the climate in the PLP" but a draft resolution which was vetoed by other permanent Security Council members could "play very badly here", he said. In the event, attempts to secure a new resolution specifically authorising war failed.
Mr Straw also warned of the uncertainty surrounding the replacement for Saddam after any war.
"A legal justification is a necessary but far from sufficient pre-condition for military action," he wrote. "We have also to answer the big question - what will this action achieve?
"There seems to be a larger hole in this than on anything."
None of the US assessments had satisfactorily answered "how there can be any certainty that the replacement regime will be better", he said, adding: "Iraq has had no history of democracy so no-one has this habit or experience."