Tony Blair is famously at his best with his back against the wall and last Thursday was no exception.

His friend and former confidant Lord Levy, the Labour fundraiser, is reported to have turned a shade of “white-green” when first arrested by police investigating the cash for peerages scandal in July, but not the prime minister.

Quite the opposite in fact. Sources close to Downing Street say Blair was in a bullish mood when he ushered Detective Superintendent Graham McNulty and a colleague into his ground floor “den” in Downing Street last week.

Perhaps it was the fact that his people had engineered things so that the two policemen would enter the building unseen by the media, probably via the cabinet office at the side, that had perked him up.

Or maybe it was the knowledge that the interview was taking place on the same day as 101 other carefully planned headline-grabbing announcements — including the publication of the Stevens report into the death of Diana, Princess of Wales — that gave him added zest.

Or possibly it was just sheer relief that he was to be quizzed not as a suspect under caution but as a witness who was free to go and get on with the job of running the nation at any time he pleased.

Whatever the reason, Blair is said to have struck the pose of a man in control when he made history at 11am on Thursday by becoming the first prime minister to be interviewed by police as part of a criminal investigation.

Not a great footnote for a man who entered office promising to be “whiter than white”, you might think, but if Blair was aware of what those around him were thinking he was not showing even a flash of it.

“To be honest it’s getting a bit surreal round here,” said one Downing Street aide. “It’s a bunker mentality but the prime minister keeps on smiling. He thinks he can still do it all, that he’s still in control. It’s self-delusional.”

Or is it? On Thursday Blair not only schmoozed his way through a police interview with minimum fuss but he effectively ordered Lord Goldsmith, the attorney-general, to stop a criminal investigation into the alleged bribery of Saudi Arabian officials by BAE Systems, Britain’s biggest arms manufacturer.

Again the popular fallout seemed minimal. While MPs accused the government of being “even more soiled than we thought it was” and of caving in to “Saudi Arabian blackmail”, a defiant Blair took “full responsibility” for the decision, saying candidly that the national interest required placing our good relations with the Saudis above the rule of law.

“I have no doubt at all that had we allowed this (police investigation) to go forward, we would have done immense damage to the true interests of this country,” Blair said in Brussels on Friday. In almost the same breath he added that it had been “perfectly natural” for a different branch of the same police force to have dropped in on him in Downing Street on the same day.

“This is a complaint that was made by the Scottish National party against me personally,” he explained to the cameras in a headmasterly tone, “and so it is not in the slightest bit surprising or wrong that police should want to talk to me.”

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