Tony Blair was warned by Jack Straw there could be post-war problems in Iraq, according to newspaper reports.
Jack Straw reportedly warned of potential post-war problems in 2002
The foreign secretary's warning came over a year before coalition forces invaded, the Daily Telegraph said.
In one letter, he told Mr Blair no-one had a clear idea of what would follow the invasion, the paper reported.
The Foreign Office said it would have been "irresponsible" not to consider these issues before the war and said Iraq was now on a path to democracy.
It came after 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.
A message said to be from their captors said they would be killed unless the US released Iraqi women prisoners within 48 hours.
The documents reportedly seen by the Telegraph were marked secret and personal and were drawn up in 2002.
They warned the prime minister of the huge difficulty faced by coalition forces after Saddam Hussein's removal from power.
According to the Telegraph, Mr Straw 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 administration 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, warned the only way to remove Saddam and his elite was to invade and impose a new government
But it said this would require "nation-building over many years" and would require "a substantial international security force", the Telegraph reported.
The newspaper said the documents warned that replacing Saddam with another "Sunni strongman" could enable troops to withdraw faster, but would run the "strong risk" of the Iraqi system "reverting to type".
And the threat of Iraq acquiring weapons of mass destruction would not necessarily be averted.
"Military coup could succeed coup until an autocratic Sunni dictator emerged who protected Sunni interests. With time, he could acquire WMD," the document said.
Even a democratic government might develop WMD so long as Israel and Iran guarded their own arsenals and Palestinian grievances remained unsolved, it continued.
According the Telegraph, the documents throw doubt on the UK government's reasons for the war.
Mr Blair was reportedly warned he would have to "wrong foot" Saddam Hussein into giving the allies an excuse for war,"
And he was told British officials believed President Bush wanted
to complete his father's "unfinished business" in a "grudge match" against Saddam", the Telegraph said.
Despite the revelations, Downing Street insisted Iraq had become a better place since the removal of Saddam Hussein.
But Tory party chairman Michael Ancram said the documents cast grave doubts on ministers' conduct in the run-up to the Iraq war.
"This underlines very starkly not only the reservations that existed in the FCO [Foreign and Commonwealth Office] about our ability to handle post-conflict Iraq but also the lack of a comprehensive plan for the stabilisation and reconstruction of Iraq, which we consistently called for before the war.
"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.
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 leaked documents were "the Crown Jewels".
He said: "They tell us that Jack Straw and others were warning there was no way to predict the consequences if Saddam Hussein was removed."
He added: "There are brutal dictators in the world but regime change is illegal.
"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."
But Commons foreign affairs committee chairman Donald Anderson told the BBC it was "reassuring" that the leaked documents showed the Foreign Office was being realistic about what might happen if Saddam Hussein was toppled.
"What we know is that the foreign secretary very reasonably pointed out we would have to be prepared to be in there for the long haul," he said.
It comes after US officials acknowledged the existence of a secret intelligence report on Iraq predicting "tenuous stability" for the country at best by the end of 2005 or else political fragmentation and civil war.
And many analysts in Washington are raising doubts about the viability of holding elections in Iraq in January if the current escalation of violence does not abate.