When, around 6.30am on Friday, four officers from Scotland Yard turned up on Ruth Turner’s doorstep, the “cash for honours” investigation entered a new phase in two important respects. The first was that Ms Turner, one of Tony Blair’s closest aides, was arrested not just under the Honours (Prevention of Abuses) Act 1925 but also “on suspicion of perverting the course of justice”. The second was that within an hour of the news of her arrest breaking, the prime minister issued a statement declaring that “Ruth is a person of the highest integrity” and that he had “complete confidence” in her.

For those outside Assistant Commissioner John Yates’s tight circle in Scotland Yard, it is impossible to know what evidence the police have against Ms Turner and whether charges will ever be brought. Perverting the course of justice is, however, a serious crime. It was the suspicion of that crime that persuaded the police that it was necessary to turn up at her address at dawn rather than invite her along for a cup of tea at the station at her leisure.

That is why Mr Blair’s allies, protesting at police heavyhandedness, risk egg on their faces. David Blunkett, who as home secretary would have hit the roof if the police had treated suspected criminals with kid gloves, called for “thoroughness, not theatre”. Tessa Jowell, the culture secretary, said she was “slightly bewildered” by the arrest, given that Ms Turner had fully co-operated with the police in the past. As the boys in blue get closer to the prime minister, his praetorian guard are turning fire on the Metropolitan police. Hold tight, because we can expect gunfire.

What might perverting the course of justice mean here? There is a history, from Watergate onwards, of the clincher being the cover-up, not the original crime. If the police suspect information relevant to their inquiry has been destroyed (irrespective of whether it would have proved their case), they may have the basis to proceed. The public will not dwell on the niceties. If an aide this close to Mr Blair, or indeed any others inside No 10, is charged, voters will make up their own minds about where the buck should really have stopped. That will be disastrous for the prime minister’s reputation and, despite an imminent change of leadership, for Labour.

That is why Mr Blair is raising the stakes. When he has been asked about the investigation in the House of Commons he has said that it would be inappropriate to comment. On Friday, for the first time, he spoke out. The arrest of Lord Levy, his chief fundraiser, came and went without a statement, so why one in support of Ms Turner? Was it that he feels an injustice is being done, or is he just scared stiff that the investigation has got too close and too dangerous? Mr Blair has already become the first serving prime minister to be questioned by police in the context of a criminal inquiry. Downing Street is amassing its forces against the Met. Having first laughed off the investigation as an irrelevance that would run into the sand, Mr Blair’s allies are now trying to intimidate the police, who appear to have acted with admirable thoroughness, with accusations of being overzealous.

Events are further complicated by the squabble between Lord Goldsmith, the attorney-general, and Lord Falconer, the lord chancellor. The latter had suggested that Lord Goldsmith would stand aside from advising the Crown Prosecution Service on whether to prosecute. On Friday the attorney-general said he would do no such thing. As things stand, Mr Blair and his mob are in full cry against the police while one of his closest political allies will advise on prosecution.

If at the end of all this Lord Goldsmith rules that there is no case to answer, nobody will be convinced. Angus MacNeil, the Scottish Nationalist MP whose complaint sparked the inquiry, said: “Water is lapping around Blair’s neck.” Even a skilled Houdini like the prime minister will find it hard to swim entirely free from this.