Tony Blair's notorious promise to former US president George Bush that "I will be with you whatever" over Iraq should have been shown to Cabinet before being sent, Whitehall's top civil servant has told MPs.
The vow was contained in a private memo sent to Mr Bush in July 2002 - eight months before Parliament voted for war - and was highlighted by this year's Chilcot report as a key moment when policy on Iraq was made without collective ministerial discussion.
Cabinet Secretary Sir Jeremy Heywood told a committee of MPs that any such documents should be subject to the approval of Cabinet.
Pressed over whether the principle of collective responsibility was observed in the run-up to the Iraq War, he replied: "Not at all times, no."
Giving evidence on the lessons to be learnt from the Chilcot report, Sir Jeremy told the House of Commons Public Administration and Constitutional Affairs Committee: "I certainly agree that private memos from the prime minister to the president of the United States setting out what the government's position is should have been subjected to collective approval, and would be today."
He added: "I think the cabinet secretary in that situation should seek a one-on-one meeting with the prime minister to speak to them directly and say they really must share this collectively if it is going to become government policy.
"That's the way the rules of Cabinet work."
If a prime minister refuses to share such a document with Cabinet - as Mr Blair did - then the civil service chief has no sanction other than to resign his post, he added.
Sir Jeremy said that a review was under way within Whitehall about the lessons that should be learnt from the Chilcot Inquiry, which should be completed within months.
He defended the length and cost of the inquiry, which took almost seven years to report after being commissioned by former prime minister Gordon Brown in 2009 and left taxpayers with a bill of more than £10m.
The inquiry was "considerably cheaper" than other probes of a similar length, he told the committee, adding: "I don't think we should put a money constraint on it. Ten million pounds - if that's what it costs, that's what it costs."
