In his Telegraph column today, Boris Johnson casually proposes dispensing with one of the fundamental principles of our legal system: the presumption of innocence. The Mayor of London calls for "a swift and minor change so that there is a 'rebuttable presumption' that all those visiting war areas [Iraq and Syria] without notifying the authorities have done so for a terrorist purpose".
It is worth revisiting, then, a column he wrote in the same paper in 2005 denouncing Tony Blair's plan to "take away our 800-year-old freedoms". In response to the proposed detainment of terrorist suspects without trial, he argued that "it must remain an inalienable principle of our law that, if the state has enough evidence to incarcerate someone, then it must have enough evidence to put him on trial".
We all agree with the Prime Minister that we face a new and sinister peril in al-Qa'eda. But it is worth pointing out that many British lives were lost every year in the 1980s and 1990s to Irish republican terror - far more than have been lost, since 2001, to the operatives of al-Qa'eda - and it is more than 30 years since we last tried interning IRA suspects.
Why is Mr Blair assuming these astonishing powers now? The Government says it must suspend the right to a trial, because trials would jeopardise the secret telephone intercepts of the security services. That sounds like phooey.
The Tories have offered a sensible compromise, by which a pre-trial judge would hear any sensitive evidence, and decide what needed to be screened out, without prejudicing the fairness of a full trial. Why can't Blair accept that? As my colleague Richard Shepherd said yesterday in the Commons, there is something deeply un-British about this Bill, something profoundly ignorant of the history of this country and the rights of its people.
Of course terrorist suspects should be rounded up. Of course we should crack down hard on anyone to do with al-Qa'eda. But it must remain an inalienable principle of our law that, if the state has enough evidence to incarcerate someone, then it must have enough evidence to put him on trial.
But in the face of understandable outrage over Isis, the Mayor has apparently decided that we can do without the right to a fair trial after all. His new stance is further evidence, as I have written before, that he is a Marxist of the Groucho variety: "Those are my principles, and if you don't like them ... well, I have others."