The Daily Telegraph reported on Saturday Foreign Secretary Jack Straw sent a letter in March 2002 marked "secret and personal" to Blair, seen by the paper, warning no one had prepared for what could happen afterwards.
"There seems to be a larger hole in this (what would happen afterward) than anything," the daily quoted Straw as saying.
"No one has satisfactorily answered how there can be any certainty that the replacement regime will be any better. Iraq has no history of democracy so no one has this habit or experience."
Downing Street on Saturday said it would not comment on the leaked documents but, added the government "firmly believes that Iraq is a better place for the removal of Saddam Hussein."
If found to be true, however, the leak is a damaging blow for Blair and shows the depth of opposition in his government to joining the US-led invasion in March 2003.
The paper said senior ministerial advisers warned in a "Secret UK Eyes Only" paper that success would only be achieved if the United States and others committed to "nation building for many years".
"No one has satisfactorily answered how there can be any certainty that the replacement regime will be any better"
Jack Straw, as quoted by The Daily Telegraph
"The greater investment of Western forces, the greater our control over Iraq's future, but the greater the cost and the longer we would need to stay," it said.
Blair built a case for war on the basis that Baghdad possessed banned weapons of mass destruction, although no biological or chemical weapons have been found following Saddam's overthrow.
But the Daily Telegraph said British officials believed US President George Bush instigated war because he wanted to complete his father's "unfinished business".
"Even the best survey of Iraq's WMD (Weapons of Mass Destruction) programme will not show much advance in recent years," a Foreign Office policy director said.
A UK official called the invasion
a Bush-Saddam 'grudge match'
"Military operations need clear and compelling military objectives. For Iraq 'regime change' does not stack up. It sounds like a grudge match between Bush and Saddam."
The Foreign Affairs spokesman for Britain's third political party, the anti-war Liberal Democrat's Menzies Campbell, said, if accurate, the letters provided a "devastating insight" into the political run up to war.
"They demonstrate that the government agreed with the Bush administration on regime change in Iraq more than a year before military action was taken," he said.
"The British government has not come clean and been frank with the British people, either about regime change or the long-term troop commitment which would result if Saddam was removed."
Bush's father was president during the first Gulf War when a US-led coalition freed Kuwait in 1991 and then drove Saddam's forces back deep into Iraq before withdrawing.