Comment: If this were true would authorities really allow Blair to sit next to the Queen?

Tony Blair defied an assassination threat from al-Qa'eda to take part in the Queen's Golden Jubilee celebrations in central London, it can be revealed for the first time.

The risk to the Prime Minister was disclosed to a new BBC documentary by Lord Stevens, the Metropolitan Police Commissioner in 2002. He positioned marksmen around Buckingham Palace in readiness for an attack by bombers or snipers.

Lord Stevens, talking for the first time about the alleged plot, said: "There was a threat against the Prime Minister over the Queen's Jubilee period. It was an assassination threat. There was good reason to believe that the threat was credible."

Mr Blair had become a target after backing the overthrow of the Taliban in Afghanistan by the deployment of British air power and troops. He was also using his diplomatic influence to try to generate support for President George W Bush to invade Iraq.

The climax of the celebrations in the summer of 2002 was a street party in The Mall attended by the Queen and senior members of the Royal Family.

The question for Scotland Yard was whether it was advisable for the Prime Minister to take his seat with the dignitaries in front of the palace.

"I certainly went to No 10 to speak to the Prime Minister and talked about public appearances," said Lord Stevens. "He was absolutely determined that he was going ahead with the public appearances and said, 'John, I am quite sure that you will do everything necessary as you have done in the past'."

Lord Stevens, who confessed to not feeling the same degree of optimism, said that Mr Blair would not countenance withdrawing. "We went ahead. There were a lot of police officers around. There were lots of marksmen in the actual vicinity," he said.

"I remember sitting with him three or four along the line outside Buckingham Palace where I knew we were extremely vulnerable. I just looked out the side of my eye. There was not a shadow of any kind of fear. He had a job to do and he did it.

"You have to admire him and Cherie Blair, who also knew about the threat, for doing that. They showed great personal courage."

The documentary, the second in a three-part series, examines how Mr Blair moved from expressing the hope in 1997 that in his lifetime the Armed Forces would not be involved in any armed conflicts to committing them to fighting in Kosovo, Afghanistan, Sierra Leone and Iraq within six years.

Clare Short, the former international development secretary, said: "Tony got the taste for war in Kosovo. It made him an heroic figure. I think he was right. It could have been better executed. But Tony got lots of credibility and stature.

"You saw a transformation of Tony. He went on visits. Pictures taken of him in front of camps of refugees. Lots of babies called Tony were born. He was on the stage. And it was a more dramatic and powerful stage than before."

Gen Sir Mike Jackson, who was chief of the Army between 2003 and 2006, said: "When you have made that decision for the first time to commit force in pursuit of your political objectives you learn.

"You have a confidence which would not have been there on the first occasion. Having had the experience of doing it once you can draw on that and it gives you great confidence if you have to do it a second or third time."

Peter Hyman, an adviser to Mr Blair from 1994 to 2003, said that the Prime Minister found it easier to get his way on the international stage. "It is undoubtedly the case he found less constraints on the world stage," he said.

"In domestic policy you are always battling the Treasury or other departments. It is a messy, slow process. It's more clear cut on the world stage."