Those questioned said they felt Blair had become untrustworthy as far as the electorate was concerned.
The apparent suicide of scientist David Kelly, the former UN weapons inspector, has piled pressure on the embattled prime minister and sparked some calls for his resignation.
The ICM poll, published in The Guardian Newspaper, showed Blair's personal approval rating to have plunged to minus 17 from plus seven in the immediate aftermath of the war to oust Iraqi President Saddam Hussein.
The ruling labour Party has also tumbled in public opinion. The poll put Labour support at 36 percent, just two points ahead of the opposition Conservatives, compared with a 12 point lead two months ago.
Blair soldiers on
Blair is continuing with his Far East tour, despite being dogged by questions over the death of Kelly.
Speaking to Chinese students at Tsinghua University in Beijing, the British leader sidestepped a question about how he could regain the trust of the British people, saying that this was a "desperately sad" time for Kelly's family, and that a proper independent inquiry would establish what had happened.
Over the justification for war, he remained defiant towards his critics, saying he did not regret taking action against Iraq.
"No. I don't regret it. I've no doubt at all that Iraq was trying to develop those weapons. I believe, however difficult it was, that it was the right thing to do," he told a student questioner.
He said that the Iraq survey group had only just begun to do their work, and when they came to make their report "people will see what the truth is."
Blair deflected a further request - to sing a Beatles song. His wife Cherie took up the challenge and obliged with a verse of "When I'm 64".
Blair received a warm welcome from Chinese Premier Wen Jiabao on Monday, and China and Britain vowed to work together to produce a "stable, prosperous and peaceful" world.
The meeting between the two leaders was dominated by North Korea's quest for nuclear weapons and Hong Kong's controversial proposed anti-sedition law.
Iraq's reconstruction and tension between India and Pakistan were also up for discussion.
"What's interesting is the degree to which the Chinese leadership here now are very much trying to assist in resolving some of these most difficult questions," the prime minister told reporters.
Later in the day, Blair met President Hu Jintao and retired senior leader Jiang Zemin.
Publicly, the mood could not have been more positive, with Wen
crediting Blair personally for putting Sino-British ties back on track after the rows prior to Hong Kong's handover in 1997.
"Since you became the prime minister, China-UK relations have
achieved much progress," said Wen, who was meeting Blair in Beijing for the first time since an overhaul of China's top leadership.