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Intelligence gathered from sources is central to the Butler inquiry's probing of the accuracy of what was known about Iraqi weapons of mass destruction.
The report says key intelligence was unreliable and criticises the MI6's validation of what it learned.
MI6 lacked agents with first-hand knowledge of any nuclear, chemical, biological or ballistic programmes, the report said.
And it said the Chinese whispers-style chain of reports diluted intelligence.
Lord Butler acknowledged Iraq was a "very difficult" intelligence target, even more so since the departure of United Nations weapons inspectors in 1998.
But he and his team criticised the way information that was available from sources was validated by MI6. Much of it has been thrown into doubt since the war.
MI6 gave more credence to untried agents than would usually be the case because of the scarcity of information. And "speculative" reports came from agents reporting from beyond their usual territory.
The report said the brutalities of Saddam Hussein's regime, with ruthless counter-intelligence and severe punishments, made it hard to find and keep reliable agents.
And 1990s budget cuts meant there were less case officers and a weakened internal process to ensure MI6 agents' quality.
In the future, Lord Butler insisted, it was of "vital importance" human intelligence sources were effectively scrutinised.