The memo predicted the invasion of Iraq would lead to a "protracted and costly" post-war occupation of the country, The Washington Post reported on Sunday.
"We disagree with the characterisation. There was significant post-war planning," said David Almacy, a White House spokesman.
"More importantly, the memo in question was written eight months before the war began - there was significant post-war planning in the time that elapsed," he said.
The memo showed that top British officials thought the Bush administration would inevitably decide to go to war, but said "little thought" had been given to "the aftermath and how to shape it", the Post said.
"A post-war occupation of Iraq could lead to a protracted and costly nation-building exercise"
Blair's staff produced the eight-page memo on 21 July 2002, in preparation for the prime minister's meeting with his national security staff two days later at Downing Street.
"A post-war occupation of Iraq could lead to a protracted and costly nation-building exercise," the memo said.
"As already made clear, the US military plans are virtually silent on this point. Washington could look to us to share a disproportionate share of the burden," it said, according to the Post.
"Some things we prepared for did not happen, like large numbers of refugees needing humanitarian assistance," Almacy said. "And others we did not expect, such as large numbers of regime elements fleeing the battlefield only to return later."
The toll in Iraq continues to mount. The United States, which led the invasion in March 2003, has said it will not pull out until Iraqi forces are trained to take over security for their country.
British officials thought the Bush
administration would go to war
The report of the 21 July memo comes after the minutes of the subsequent Downing Street meeting were published by London's Sunday Times on 1 May and became known as the Downing Street memo.
The minutes said Britain's spy chief had concluded after a trip to Washington that "intelligence and facts were being fixed" to make the case for war in Iraq, an assertion that US officials and Blair have denied.