Tony Blair was warned by Jack Straw there could be post-war problems in Iraq, according to newspaper reports.
The warning from the Foreign Secretary came more than a year before coalition forces invaded the country, 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.
Downing Street said it never commented on leaked documents, but insisted Iraq was now a better place.
The documents reportedly seen by the newspaper 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: "There seems to be a larger hole in this than anything.
"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."
Some experts not working for the government predicted before the war that Iraq would descend into chaos, with battles lines drawn along ethnic and religious lines and a guerrilla warfare campaign being waged against the coalition.
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.
"They demonstrate that the government agreed with the Bush administration on regime change in Iraq, more than a year before military action was taken.
"The justifications offered on many occasions to Parliament and the public that the issue was one of WMD are shown to be a mechanism designed to get round the legal obstacles in international law against the removal of Saddam Hussein."
http://news.bbc.co.uk/1/hi/uk_politics/3668016.stm