WHETHER or not any charges result from the cash-for-honours investigation, it has become a thickening dark cloud over the sunset of Tony Blair's time as Prime Minister.

Yesterday's belated announcement that Mr Blair had been questioned for a second time by police leading the inquiry is an extraordinary state of affairs.

And while it must, of course, be stressed that he has again been treated merely as a witness, it remains a particularly uncomfortable position for a Prime Minister to be in.

The sense of intrigue and mystery has been heightened by the peculiar demand from the police for a week-long news blackout of the interview with Mr Blair for "operational reasons".

What on earth could those operational reasons be? Could the interview have been linked to the second arrest a few days later of Labour's chief fundraiser Lord Levy?

It is all having a profound effect on Mr Blair's last days in power and is creating further unrest among his own backbenchers for a resignation timetable.

But, while David Cameron can hardly be blamed for demanding Mr Blair's resignation, the on-going cash-for-honours affair actually makes it less likely that the Prime Minister's date of departure from Number 10 will be brought forward.

It would look for all the world like an admission of guilt and - until proven otherwise - Mr Blair is nothing more than a witness in an ongoing police inquiry.