Powell's comments, revealed in an inquiry into the suicide of weapons expert David Kelly, cast further doubt on Blair's own claim in the foreword to the dossier that Iraq's biological and chemical weapons programme posed a "serious and current threat".
His e-mail made clear the evidence alone would not turn sceptical public opinion, saying: "The dossier is good and convincing for those who are prepared to be convinced."
Senior judge Lord Hutton's inquiry is a key test for Blair, whose public trust ratings have plunged over the government's handling of the Kelly affair and the failure to find any banned weapons in Iraq four months after Saddam's overthrow.
Kelly slashed his wrist after being named as the source for a BBC reporter who accused Blair's communications chief Alastair Campbell of "sexing up" the dossier by inserting claims that Saddam could deploy banned weapons at 45 minutes' notice.
The inquiry also saw an e-mail between Campbell and Powell, dated September 5, in which Campbell said a decision was taken for a "substantial rewrite" of the dossier and a restructuring of the document "as per TB (Tony Blair's) discussion".
That will give fresh ammunition to critics who argue that Blair's officials tried to unduly influence the intelligence services' presentation of evidence against Saddam.
A poll last week showed 41 per cent of the British public blame the government for Kelly's death and 68 per cent think the government was dishonest over the Iraq war.
Blair released a series of pre-war documents to bolster the case for military action. The September dossier has been criticised over the 45-minute claims, which came from a single uncorroborated source, and disputed allegations that Iraq had sought to acquire uranium from Niger for nuclear weapons.
A later "dodgy" dossier detailing Iraq's efforts to deceive UN weapons inspectors included large chunks lifted from a student thesis. A third dossier on human rights was filled with cases which were ignored when Saddam was an ally of the West.
Powell, in his note to Joint Intelligence Committee chief John Scarlett, said the government should make clear "we do not claim that we have evidence that (Saddam) is an imminent threat" when it published the September dossier.
Many of Blair's own Labour Party parliamentarians, who only reluctantly backed military action, say the government did exactly that by playing up the 45-minute claims in the dossier.
Blair, currently on holiday in Barbados, is due to return to give evidence to Hutton's inquiry. Campbell is expected to take the stand on Tuesday and, along with Powell, will be the closest of Blair's advisers to be questioned about the most damaging crisis of the Labour leader's six-year rule.
- REUTERS
Hutton inquiry website
British Parliament Foreign Affairs Committee transcript:
Evidence of Dr David Kelly
Key players in the 'sexed-up dossier' affair
Herald Feature: Iraq
Iraq links and resources