On 26 February 2003, Tony Blair faced the largest backbench rebellion of his premiership up to that point over the government’s policy towards Iraq.

Following a fraught six-hour House of Commons debate, a much larger than anticipated 121 Labour MPs defied a three-line whip to support an amendment stating that the case for military action was “as yet unproven”.

The vote came on a cross-party amendment to a government motion, tabled by former cabinet ministers Chris Smith (Labour) and Douglas Hogg (Conservative).

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In November 2002, the UN Security Council unanimously passed resolution 1441 to give Iraq “a final opportunity to comply with its disarmament obligations”.

Some months later on 25 February 2003, the US and the UK submitted a draft resolution to the UN arguing that Iraq had missed its “final opportunity” to disarm peacefully. It declared that Iraq had “failed to take the final opportunity afforded to it in resolution 1441.” France, Germany and Russia submitted a rival proposal to the council calling for further weapons inspections as an alternative to military action.

The US and UK governments did not submit the resolution to a vote immediately, instead choosing to give Iraqi dictator Saddam Hussein one further chance to disarm.

Tony Blair, the prime minister, delivered a statement to the House of Commons on the nature of the new UN draft resolution. Blair pointed to 12 years of “bitter experience”, stretching back to 1991 as proof that peaceful disarmament could only happen “with Saddam’s active co-operation”.

Blair denied that the US and UK were “rushing to war”.

He told the commons: “We are now 12 years after Saddam was first told by the UN to disarm; nearly six months after President Bush made his speech to the UN accepting the UN route to disarmament; nearly four months on from resolution 1441; and even now, today, we are offering Saddam the prospect of voluntary disarmament through the UN.”

The prime minister also warned that failing to act would destroy the authority of the United Nations and leave Saddam with his “weapons of mass destruction intact”.

He argued that inaction would not amount to “a road to peace, but folly and weakness that will only mean that when the conflict comes, it is more bloody, less certain and greater in its devastation.”

Iain Duncan Smith, the Conservative leader, expressed support for the government’s stance. However, he warned that the resolution risked being vetoed in the United Nations Security Council, given the rival proposal advanced by France, Germany and Russia.

The statement set the stage for the debate the following day, on 26 February.

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The commons debate was held on a motion referencing the UN draft resolution.

It stated: “That this House takes note of Command Paper Cm 5769 on Iraq; reaffirms its endorsement of United Nations Security Council Resolution 1441, as expressed in its Resolution of 25th November 2002; supports the Government’s continuing efforts in the United Nations to disarm Iraq of its weapons of mass destruction; and calls upon Iraq to recognise this as its final opportunity to comply with its disarmament obligations.”

Smith and Hogg’s amendment proposed to leave out the words after “destruction”. The amended motion would, therefore, no longer have endorsed the implication that Iraq was facing a last warning before facing military action.

Crucially, the amendment proposed to add the words: “…but finds the case for military action against Iraq as yet unproven.”

Jack Straw, the foreign secretary, said the original motion did not amount to “an endorsement of military action by United Kingdom forces.”

He described a “moment of choice” for the UN, referencing the failure of its inter-war predecessor organisation, the League of Nations.

Straw said: “Yet the League failed because it could not create actions from its words: it could not back diplomacy with a credible threat and, where necessary, the use of force. Small evils therefore went unchecked, tyrants became emboldened, then greater evils were unleashed.”

Smith, moving the rebel amendment, said he could not support an amendment that “endorses the timetable that is now upon us – which will lead, I fear, inexorably to war within the next three to four weeks.”

He told the House: “Let us not forget what we are talking about. We are talking about going to war.

“We are talking about thousands – possibly hundreds of thousands – of innocent lives being lost. We are talking about casualties almost inevitably among our own forces. We are talking about instability across the whole Middle East.

“One does not undertake such things lightly. One must have the clearest possible reasons for doing them, and I do not believe that those reasons are there.”

Hogg argued that war could only be moral if the threat was “imminent and grave”, adding: “In all conscience, I do not think that we can say that about Iraq.”

Conservative MP Kenneth Clarke said that he could not rid himself of “doubts that the course to war upon which we are now embarked was decided on many months ago, primarily in Washington.”

In the end, the rebel amendment was rejected by 393 votes to 199.

The original motion was ultimately passed unamended by 434 votes to 124.

However, the internal revolt on the cross-party amendment comprised nearly a third of the parliamentary Labour Party (PLP). Previously, the biggest revolt of Blair’s premiership came after 67 Labour MPs voted against cuts to disability benefits in 1999.

This time, 121 Labour MPs rebelled – a larger than anticipated number. They were joined by 13 Conservatives, 52 Liberal Democrats and 13 MPs from other smaller parties in supporting the amendment.

In the final vote, 59 Labour MPs voted against the government. The Labour rebels included the father of the house, Tam Dalyell, and former ministers Glenda Jackson, Peter Kilfoyle and Mark Fisher.

Graham Allen, a leading Labour rebel, said the vote was proof that Blair needed to “listen to parliament and respect the view of the British people that we should get off George Bush’s escalator to war.”

Charles Kennedy, the Liberal Democrat leader, stated: “This is a very significant parliamentary occasion. Despite investing masses of political and parliamentary capital, the government has still failed to persuade a third of the House of Commons.

“At this crucial stage, that sends a potent signal to the government of both Britain and the United States.”

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On 10 March, France announced it would join Russia in vetoing a re-drafted UN resolution which gave Iraq until 17 March to disarm. Britain, the United States and Spain, having all promoted the proposed resolution, then abandoned it in the face of opposition.

Blair and Bush had failed to secure a “second resolution”, which was seen as crucial in justifying the march to war.

The attorney general, Lord Goldsmith, provided his final piece of public advice to parliament in a written answer on 17 March.

It stated that the “Authority to use force against Iraq exists from the combined effect of Resolutions 678, 687 and 1441. All of these resolutions were adopted under Chapter VII of the UN Charter which allows the use of force for the express purpose of restoring international peace and security.”

On 18 March, following one of the most momentous debates in modern parliamentary history, the House of Commons voted to authorise military action against Iraq.

Less than 24 hours after the commons debate, the US-led Coalition Forces launched an air campaign against Iraq, followed the next day by a ground invasion. British military action began on 20 March.

Blair notified the nation with a live televised address.

On 6 July 2016, the findings of the Iraq Inquiry, chaired by Sir John Chilcot, were published after seven years of investigation. The 2.6 million-word report – longer than the King James Version of the Bible, the complete works of William Shakespeare, and Tolstoy’s War and Peace put together – provided a searing indictment of the intelligence failures, shaky legal rationale, miscalculations, inadequate military preparation and exaggerated public statements that led to the UK’s involvement in the 2003 invasion of Iraq.