Tony Blair's policy on Iraq was made on the basis of "flawed intelligence" and the process for deciding that the 2003 invasion was legal was "far from satisfactory", a long-awaited report into the conflict has found.
The Iraq Inquiry found that Mr Blair's government presented evidence about Saddam Hussein's weapons of mass destruction (WMDs) "with a certainty that was not justified" and troops were sent in before all peaceful options had been exhausted.
Presenting a summary of his inquiry's findings, Sir John Chilcot hit out at the "wholly inadequate" planning for the period after the fall of Saddam, which saw British troops involved in a prolonged and bloody occupation.
The former Whitehall mandarin was setting out the findings of his inquiry into the UK's most controversial military engagement since the end of the Second World War.
Although his inquiry did not express a view on whether the invasion was legal, Sir John criticised the way in which Mr Blair and his attorney general, Lord Goldsmith, had reached their decision on the legal basis.
Sir John said: "The circumstances in which it was decided that there was a legal basis for military action were far from satisfactory."
Addressing the issue of Saddam's WMDs, which were used as the justification for the war, Sir John said the Joint Intelligence Committee should have made clear to Mr Blair that the intelligence had not established "beyond doubt" that Iraq had either continued to produce chemical and biological weapons or was continuing with efforts to develop a nuclear bomb.
"It is now clear that policy on Iraq was made on the basis of flawed intelligence and assessments," Sir John said. "They were not challenged, and they should have been."
Sir John also indicated that, by acting without the majority support of the UN's Security Council for military action, the UK was "undermining" its authority.
The inquiry found:
:: Judgments about the severity of the threat posed by Iraq's weapons of mass destruction - WMD - were "presented with a certainty that was not justified";
:: There was "little time" to properly prepare three military brigades for deployment in Iraq, the risks were not "properly identified or fully exposed" to ministers, resulting in "equipment shortfalls";
:: Despite explicit warnings, the consequences of the invasion were under-estimated;
:: Planning and preparations for Iraq after Saddam's fall were "wholly inadequate";
:: Mr Blair's government failed to achieve its stated objectives.
Sir John said taking part in the US-led invasion of Iraq was "a decision of the utmost gravity" but acknowledged that Saddam "was undoubtedly a brutal dictator who had attacked Iraq's neighbours, repressed and killed many of his own people and was in violation of obligations imposed by the UN Security Council".