Tony Blair must take responsibility for any intelligence failings identified in the Butler Report, the former head of the Joint Intelligence Committee says.
Dame Pauline says "the buck must stop" with Mr Blair
Dame Pauline Neville-Jones said the "buck must stop" with Mr Blair if Wednesday's report finds failings.
She told BBC World's Hardtalk programme the prime minister should acknowledge his mistakes in the run-up to the war.
Dame Pauline also suggested new MI6 head John Scarlett should consider his position if criticised by Lord Butler.
Mr Scarlett drew up the government's Iraq weapons dossier as chairman of the Joint Intelligence Committee (JIC).
Mr Blair has been criticised for appointing him to head MI6 before seeing the Butler report's conclusions.
According to the Independent, Mr Scarlett is among those who have been sent letters outlining sections of the report that will criticise them directly.
Dame Pauline said that she did not want to "pre-judge" Lord Butler, but that if the report criticised Mr Scarlett it would raise question marks over his suitability to head the intelligence agency.
Her comments came on the day US senators criticised American intelligence agencies, saying the Iraq war had been based on "flawed" information.
Dame Pauline drew comparisons between Mr Blair and former CIA Director George Tenet, who resigned amid criticism of the quality of CIA intelligence in the run-up to the Iraq war.
Lord Butler's findings are due to published on Wednesday evening
"The head of the CIA is a politically appointed job, so it's the equivalent of the minister going," she said.
She said the failure within British agencies did not let Mr Blair off the hook.
"The buck stops there and I don't think that the political layer in any country can escape the consequences of a systemic failure," she said.
She refused to say whether Mr Blair should resign if the report found he had been wrong, but said: "He is making a distinction between what he genuinely believed and what turns out to be the case so he is at least open to the accusation of incompetence."
On Saturday Dame Pauline stressed that any failings of the intelligence services in Iraq would not mean all their other work was substandard or inadequate.
"We need to be careful about assuming that the report is going actually to say that everything these agencies are doing needs revision.
"I do think it will want to recommend certain ways in which you can make your assessment process more robust."
Speaking on BBC Radio 4's Today programme on Saturday the former foreign secretary Robin Cook said the suggestion that there had been a "global" intelligence failure was "garbage"
"Nobody except Washington and London thought that Saddam was such a threat
that we had immediately to go to war," he said.
"The governments had made up their mind that Saddam had weapons and must be a threat, they had made up their mind they were going to go to war.
"The intelligence agencies were then left in a position of having to find evidence to support a conclusion."
Mr Cook said John Scarlett had told him before the war that Saddam Hussein did not have any weapons of mass destruction that could be fired over long distances at strategic cities.
"I still find it perplexing why Number 10 came to a different conclusion.
"When that report comes out it is very important that the response to it does not try to brush under the carpet the fact that there were very serious errors made, that we ended up going to war on intelligence that turned out to be unreliable - whoever is to blame."
Mr Cook told the programme the prime minister would agree he should take responsibility for any failings found by Lord Butler.
"He has never in any way tried to walk away from the fact that the decision to go to war was a very personal one of his own and he fully accepts that responsibility.
"He needs an opportunity - and perhaps the Butler Report will give him that - to say that 'look, there were mistakes made, but lessons have been learnt' but above all he needs to be able to say ' and it's not going to happen again'.