Tony Blair has always maintained that Iraq's oil reserves did not cross his mind in the run-up to the 2003 invasion. But Iraq's abundant oil supplies had certainly crossed the minds of the British energy giant BP. As this newspaper reported yesterday, documents reveal that the oil company discussed Iraq's fossil fuel reserves in considerable detail with government officials in late 2002.

Indeed, BP was pressing for a share of the spoils in the wake of "regime change" in Iraq. When the company feared it was being excluded from agreements made in Washington to carve up access to Iraq's oil supplies, the former trade minister Baroness Symons agreed to lobby the Bush administration on BP's behalf.

This is a significant finding because it casts doubt on Mr Blair's claim that his government was a disinterested party when it came to the question of Iraq's oil wealth. But it is also significant because, at the time these documents were written, the former Prime Minister was still insisting that Saddam Hussein could remain in power if he co-operated with United Nations weapons inspectors. It is hard to reconcile the claim that no decision on military action had been taken with these extensive behind-the-scenes preparations to divide access to Iraq's natural resources.

Prior to the invasion, Mr Blair said that the idea that the invasion was, in any way, motivated by Iraq's oil was "absurd". He argued in a debate hosted by BBC's Newsnight in February 2003 that if oil was the West's goal, it could just as easily have cut a deal with Saddam. Yet this does not serve to refute the argument that oil was a motivation behind toppling the Iraqi dictator. It is perfectly possible that Western powers anticipated getting access to Iraq's oil on favourable terms after the removal of Saddam.

This is not fanciful speculation. The US was conspicuously slow to hand over power to an Iraqi civilian government in Iraq after the fall of Baghdad. Paul Bremer's Coalition Provisional Authority was only disbanded after protests by the dominant Shia community. The US gave every impression that it wanted to continue pulling the strings in Iraq, including over the awarding of energy contracts. Alan Greenspan, the former chairman of the US Federal Reserve, for one, has reached the conclusion that the Iraq war was "largely about oil".

As for Mr Blair, the former Prime Minister was certainly not shy about championing the interests of British oil companies around the world. When he met Muammar Gaddafi in the Libyan desert in 2007, Mr Blair saw fit to take the then chairman of BP, Peter Sutherland, along with him. A few days later, BP signed a $900m exploration deal with the Libya Investment Corporation. Realpolitik was entwined with access to fossil fuels in Libya. It is no great stretch to imagine that there were similar motivations over Iraq.

These documents do not prove that the British government's invasion was primarily motivated by a commercial desire to profit from Iraq's oil. Indeed, it is probably too simplistic to present any single factor as the decisive motivation for this calamitous misadventure. But they do make it clear that, contrary to ministerial denials, oil was something that ministers were thinking about in those months prior to the invasion.

These documents were not, apparently, reviewed by the Chilcot inquiry, which has been tasked with investigating Britain's involvement in the Iraq invasion. That is manifestly unsatisfactory. No independent inquiry into the Iraq imbroglio can be complete without taking into account the influence of Big Oil. Unless Chilcot takes this important new evidence into consideration, it is hard to see how its conclusions can be credible.