The documents themselves - transcripts of interviews from an internal defence ministry review of the conflict - disclose that some planning for the Iraq war had begun a year and a half before the invasion.
Andrew Gilligan, the reporter who obtained the documents, told Al Jazeera on Sunday said his report showed Blair "didn't feel able to be honest with the British people.
"The fact is, throughout 2002 as the Americans were quite openly preparing for war, Tony Blair was saying no decisions had been made, that Britain was not making any preparation of its own, that it was a matter for the United Nations.
"And we now know from these documents that military planning in Britain had started as early as February 2002."
In July 2002, Blair told lawmakers at a House of Commons committee session that there were no preparations to invade Iraq.
"These documents show that planning [for the invasion of Iraq] began as far back as 2002"
Scottish Nationalist leader
Critics of the war have long insisted that Blair offered then-President George Bush an assurance as early as mid-2002 -before British lawmakers voted in 2003 to approve UK involvement - that Britain would join the war.
The leaked documents are likely to be supplied to a public inquiry established by Prime Minister Gordon Brown to scrutinize prewar intelligence and postwar planning, and which will hold its first evidence sessions later this week.
Brown appointed ex-civil servant John Chilcot to lead the panel, which will call Blair and the current and former heads of Britain's MI6 intelligence agency - John Sawers and John Scarlett - to give testimony in person.
According to the Sunday Telegraph, military leaders used the defence ministry review to criticize government departments over their failure to plan for reconstruction work once Saddam Hussein had been deposed.
"We got absolutely no advice whatsoever. The lack of involvement by the FCO (Britain's Foreign and Commonwealth Office), the Home Office and the Department for International Development was appalling," the newspaper quoted Brigadier Bill Moore as saying in his statement.
The newspaper said the internal review concludes that a swift military victory was won only because Iraq's forces were so poor. "A more capable enemy would probably have punished (our) shortcomings severely," it quotes a document as saying.
Alex Salmond, leader of the Scottish Nationalist Party, said the revelations prove Blair took Britain to "an illegal and disastrous war on false pretences".
"Tony Blair consistently denied to Parliament and public that the UK government was preparing for war in Iraq, yet these documents show that planning began as far back as 2002,"
The defence ministry declined to comment Sunday on the leaked documents, but said it "recognizes the importance of identifying and learning lessons from operations."
Two previous British studies into the war have been carried out .
One cleared the government of blame for the death of David Kelly, a government weapons scientist who killed himself in 2003 after he was exposed as the source of a BBC report that accused Blair's office of "sexing up" prewar intelligence.
A separate 2004 inquiry — which Chilcot took part in — into intelligence on Iraq also cleared Blair's government, but criticized spy agencies for relying on seriously flawed or unreliable sources.
Findings of the new inquiry will not be published before next summer, meaning the conclusions won't be known before Britain's next national election, which Brown must hold by June 2010.