The Tories have accused Prime Minister Tony Blair of going to war in Iraq without a comprehensive plan to restore normality to the country.
Jack Straw reportedly warned of potential post-war problems in 2002
The Daily Telegraph published leaked papers suggesting the prime minister was warned in 2002 that an invasion could lead to instability.
Tory chairman Michael Ancram said they showed ministers "misled" the public.
But Mr Blair said: "The idea that we did not have a plan for afterwards is simply not correct."
He added: "We did, and we have unfolded that plan, but there are people in Iraq who are determined to stop us."
Speaking at Leeds Castle in Kent, where Ulster talks ended without a deal, Mr Blair denied he had been warned by Foreign Secretary Jack Straw that chaos would follow the ousting of Saddam Hussein.
"Having read in the papers that apparently I was warned of the chaos that was going to ensue in Iraq, I actually got the minute Jack sent to me," he said.
"It didn't do anything of that sort. What it warned of was this: it's very important that we don't replace one dictator, Saddam Hussein, with another.
"I totally agree with that."
Mr Ancram said the leaked documents dating from 2002 cast "grave doubts" on the conduct of ministers in the approach to war, and indicated "the lack of a comprehensive plan for the stabilisation and reconstruction of Iraq".
"The assurances given to us both by the prime minister and Jack Straw that such a plan was in hand were clearly misleading," he added.
In a statement, the Foreign Office said Iraq's current security situation was "serious" but it was "on a path to a democratically elected government".
"There were indeed anxieties that, after Saddam, one dictator might be replaced by another," it said.
"Not only has that not happened but for the first time Iraq is moving towards a democratic future."
According to the Telegraph, the documents show Mr Straw warned Mr Blair over a year before the invasion that no-one had a clear idea what would follow it.
Mr Straw reportedly wrote: "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."
One letter from foreign policy adviser, Sir David Manning, reportedly warned of a "real risk" that the US had underestimated these difficulties.
President Bush still had to answer big questions such as "what happens on the morning after", he reportedly warned.
Another document, compiled by the Cabinet Office Overseas and Defence Secretariat, allegedly warned "nation-building over many years" and "a substantial international security force" would be needed.
The Telegraph said documents warned of a "strong risk" of the Iraqi system "reverting to type" with a succession of military coups, should coalition forces withdraw too soon before a democratic government was established.
Mr Blair was told British officials believed President Bush wanted to complete his father's "unfinished business" in a "grudge match" against Saddam, the Telegraph said.
Liberal Democrat foreign affairs spokesman Sir Menzies Campbell said: "If these documents are accurate they provide a devastating insight into the
political run-up to war in Iraq."
He told BBC Radio 4: "The judgement which became the prime minister's judgement is one which will come back to haunt him not just today but again and again."
Earlier this week US officials acknowledged the existence of a secret intelligence report on Iraq, predicting "tenuous stability" at best until the end of 2005, or else political fragmentation and civil war.
Many analysts in Washington are raising doubts about the viability of elections in Iraq if the escalation of violence does not abate.
Meanwhile, footage of three blindfolded men, said to be a Briton and two Americans kidnapped in Baghdad on Thursday, was shown on Arab satellite channel al-Jazeera on Saturday.