Former House of Commons leader Robin Cook says he blames himself for not trying harder to persuade the prime minister against going to war with Iraq.
'Others had the chance to save Mr Blair from himself'
Mr Cook, who quit his cabinet post in March in protest at the prospect of military intervention, says he has become "increasingly angry" at his own failure to convince Tony Blair of the damage a "unilateral war" would cause him and his party.
But while he accepts that "the dynamic" for British participation in the war was driven by Mr Blair's determination to keep his "special relationship" with US President George Bush, he argues that "others" should also share the blame.
Mr Cook, who made the comments on the eve of Labour's annual party conference, also called for an "urgent return" to more collegiate government.
Cabinet and Parliament were needed to provide checks and balances "on the preoccupations" of the prime minister, he said.
In the run up to war, Mr Blair was "never challenged by a sizeable number of senior ministers", was "constantly egged on" by the Opposition and was supported by a parliament which voted to go along with him.
"As the war emerges as deeply damaging both in international diplomacy and in domestic politics, there is an increasing tendency to blame it all on Tony Blair," said Mr Cook.
"The press have already started to write that up as a major theme of next week's Labour conference.
"It is true that the dynamic for British participation came from Number 10 and was driven by Tony's determination to maintain the special relationship with George Bush.
"But it is an evasion to let Tony take all the blame. Others had the chance to save him from himself.
"He was meticulous in letting the cabinet debate Iraq, but was never challenged by a sizeable number of senior ministers.
"He agreed, in an historic precedent, that Parliament should vote on the war before troops were committed and Parliament voted to go along with him.
"The Opposition did not oppose, but constantly egged on the prime minister to go to war.
"All of us must accept responsibility for war - and I include myself.
"As I have observed from the dramatic revelations from two months of Hutton [inquiry into the death of government scientist Dr David Kelly], I have become increasingly angry with myself for not being more persuasive at repeated meetings in convincing Tony Blair of the damage he would do to himself and his party if he persisted in a unilateral war."
Mr Cook said there was "a basic dishonesty in the pretence that the Iraq Survey Group is looking for weapons" of mass destruction when their job is merely "to come up with the evidence that Saddam had ambitions to acquire them" - a fact that is already known.
Earlier this week, a source from the Bush administration told BBC journalist Andrew Neil that in interim report by the ISG, tasked with hunting for WMD, will conclude that it has not found even "minute amounts of nuclear, chemical or biological weapons material".
Downing Street immediately branded the leak as "speculation" and Foreign Secretary Jack Straw argued that just because it was "difficult to obtain physical evidence... does not mean the evidence is not there".
In his resignation speech, Mr Cook predicted that Saddam "probably had battlefield chemical munitions", although he did not recognise these as true weapons of mass destruction constituting a current and serious threat.
In his article on Friday, he admitted that even he appeared "to have overstated Saddam's weapons capacity", adding that "the cupboard is totally bare".
Mr Cook said "even the keenest advocate of the war would be hard put to claim that it has diminished terrorism".
Bomb attacks and land-mine ambushes were now so commonplace in Iraq that President Bush had designated it the new "central front" in the war against terrorism - the same country "which he had previously trumpeted as the scene of a victory against terrorism", he said.