Gordon Brown has said lessons must be learned on use of intelligence in the run-up to war, but ruled out holding an inquiry while troops remain in Iraq.
Mr Brown praised the "tremendous dedication" of troops
The prime minister-in-waiting said that in future intelligence analysis must be kept independent of politics.
The Tories and Lib Dems said it was an admission that intelligence was not properly handled or presented on Iraq.
Mr Brown, who succeeds Tony Blair on 27 June, was speaking as he made a one day "fact-finding" trip to Iraq.
Mr Brown had talks with Iraq PM Nouri Maliki on political and economic reconstruction, and met military commanders and British troops, amid tight security.
He told reporters, who were barred from reporting the visit in advance, that it was "very much an assessment more than anything else, a fact-finding trip".
The tight security for the visit appeared to be justified as a series of mortars struck the international zone in Baghdad while Mr Brown has there, although none of the entourage was injured.
Mr Brown's visit came as MPs in London debated Conservative calls for an inquiry into the Iraq war.
The chancellor said he did not back an inquiry being held while troops remained in Iraq, but agreed lessons had to be learned.
He said that in future all intelligence information must be independent of the political process, and that it must be validated and verified if made public.
He said he had asked Cabinet Secretary Sir Gus O'Donnell to look at how to ensure that independence and said he wants Parliament's Intelligence and Security Committee to have a bigger role.
The key change would be that the Joint Intelligence Committee (JIC) chairman would not be the same person as the Cabinet Office security co-ordinator. Both posts are currently held by Sir Richard Mottram, who will retire soon.
"I think it's important to learn all the lessons, just as Tony Blair has said he acted in good faith but mistakes were made. I think it's important to learn the lessons to look forward now," Mr Brown said.
But it led some to comment that the chancellor appeared to be trying to distance himself from Mr Blair and the government's 2002 dossier on Iraq's alleged weapons of mass destruction.
That was the dossier which included the now discredited claim that Saddam had weapons of mass destruction which could be used within 45 minutes of an order being given.
Lib Dem leader Sir Menzies Campbell said: "This is the first public admission by a member of the Cabinet that, before military action was taken against Iraq, there was undue influence, undue political influence, on intelligence."
And shadow foreign secretary William Hague added: "I think it is a bit of an admission that intelligence wasn't properly handled or presented at that time."
The Butler report into the accuracy of intelligence about Iraq's weapons of mass destruction found much was unreliable, but said the inquiry had seen no evidence of deliberate distortion.
Former Joint Intelligence Committee chairman Sir Paul Lever told the BBC Mr Brown's announcement was "very welcome".
"It clearly is an attempt by the next prime minister to put some distance between himself and the way in which the so-called 'dodgy dossier' and other documents were produced to justify the decision to go to war in Iraq," he said.
"It is vitally important that those people who prepare intelligence and assess intelligence are able to do their work independently and objectively."
Mr Brown has acknowledged that Iraq was a "very divisive" issue, but he has stuck by the decision to go to war.
On Monday he refused to put any timescale on withdrawing the remaining UK troops in Iraq - due to be reduced to 5,500 by mid-summer, but praised the "tremendous dedication and duty" of those he met.
Also on Monday it was announced that the tax free operational allowance for troops deployed to Iraq and Afghanistan is to be increased by 3.6% to £12.75 a day, backdated to April.
Defence chiefs say that because of the "increased stabilisation of the Balkans" personnel serving there will no longer qualify for the allowance, introduced last October, from 1 September.