The US President told Tony Blair, in a secret telephone conversation in January 2003 that he "wanted to go beyond Iraq".
He implied that the military action against Saddam Hussein was only a first step in the battle against WMD proliferation in a series of countries.
Mr Bush said he "wanted to go beyond Iraq in dealing with WMD proliferation", says the letter on Downing Street paper, marked secret and personal.
No 10 said yesterday it would "not comment on leaked documents". But the revelation that Mr Bush was considering tackling other countries over WMD before the Iraq war has shocked MPs. Saudi Arabia and Pakistan have been close allies of the US in the war against terror and have not been considered targets in relation to WMD.
The confidential memo recording the President's explosive remarks was written by Michael Rycroft, then the Prime Minister's private secretary and foreign policy adviser. He sent the two-page letter recording the conversation between the two leaders on 30 January 2003 to Simon McDonald, who was then private secretary to Jack Straw, the Foreign Secretary.
Mr Rycroft said it "must only be shown to those with a real need to know ".
The revelation that Mr Bush told the Prime Minister Iraq should be seen as a first step comes in the American edition of Lawless World, a book by the leading international lawyer Philippe Sands QC, who is also a professor of law at University College London and senior barrister at Matrix chambers, which he shares with Cherie Blair.
"The conversation seems to indicate that Iraq was not seen as an isolated issue but as a first step in relation to a broader project," he said. "What is interesting is the mention of Saudi Arabia, which to the best of my knowledge had not at that time been identified particularly as a country with WMD. An alternative view is that the mention of Saudi Arabia indicates that the true objectives were not related exclusively to WMD."
The inclusion of Pakistan, also a key US ally, is also surprising, although there has in the past been concern about nuclear proliferation in that country.
Sir Menzies Campbell, the Liberal Democrats' foreign affairs spokesman, said the timing of the conversation was significant since it took place when Britain and the US were still trying to get a second UN resolution to make the legal case for the Iraq war watertight.
"If this letter accurately reflects the conversation between the President and the Prime Minister it will cause consternation, particularly in Saudi Arabia. American policy in the Middle East for decades has been based on support for Israel and an alliance with Saudi Arabia," he said. "If this was more than loose talk and represented a genuine policy intention it constitutes a radical change in American foreign policy."
Elizabeth Wilmshurst, who resigned as the Foreign Office's deputy legal adviser because she thought the invasion of Iraq would be illegal, has questioned whether democracy and the rule of law can prevail there. She writes inThe Independent on Sunday, "there will be no prospect" of democracy, unless the Iraqis can establish the rule of law and fair trials including for Saddam Hussein.
Ms Wilmshurst said the trial, which begins this week, will be the first key test of whether the justice system can operate in Iraq.
The lawyer, who is now the Senior Fellow at the think tank, Chatham House, says: "There will be no prospect of success for the democratic process in Iraq unless the rule of law can prevail. The trial of Saddam Hussein due to start on Wednesday, presents a test."Reuse content