What does it take for a minister to resign these days? Funny you should ask, because the last time I asked it, Charles Clarke was just about to be sacked as Home Secretary in 2006. Six years ago, it was just as much a cliché as it is today that there was once a golden age when barely a week would go by without a Cabinet minister resigning on a point of honour.

The only case that is ever cited is that of Lord Carrington, who resigned as Foreign Secretary when the Falkland Islands were invaded in 1982. But that was a special case, a national humiliation comparable with the failure of the Norway campaign in 1940.

For normal foul-ups, it was never thus. But Clarke was forced to go because he let foreigners out of jail without considering them for deportation. Hunt, on the other hand, gave the go-ahead for a takeover bid that was abandoned soon afterwards.

The phone-hacking scandal scuppered Rupert Murdoch's bid for BSkyB, and created such a vacuum that David Cameron was nearly sucked into it. Because of his closeness to Murdoch, he was forced – by Ed Miliband and Nick Clegg, the alternative coalition – to set up the Leveson inquiry into media ethics.

The decision to appoint Lord Justice Leveson was a defensive chess move of the kind that makes you wonder whether politics is scripted by the gods, because it was the Leveson inquiry which disclosed the emails and texts that now threaten Hunt's career.

So, what does it take for a minister to resign? On Friday it seemed Hunt would survive, because the Leader of the Opposition said that "David Cameron looks more like a prime minister organising a cover-up rather than standing up for the public". Once you get to "looks like" you know the case is weak.

Hunt's defence is also weak: that, although he had been biased in favour of the bid before he was in charge of it, he simply followed the advice of Ofcom and the Office of Fair Trading. The trouble is that he seems to have tried to influence Ofcom's advice. There is, however, no General Theory of Ministerial Responsibility. Attempts to generalise always come up against contrary cases. Take this quiz question. Not counting general reshuffles, who was the next cabinet minister to resign after David Blunkett resigned as Home Secretary in 2004?

It was the Secretary of State for Work and Pensions, who resigned when arguably there was no need for him to do so. I was sitting next to Francis Elliott, The Independent on Sunday journalist who made the phone call to the Advisory Committee on Business Appointments that brought the minister down. The minister had failed to register a directorship while he was briefly on the backbenches.

It was a breach of the rules, but a stronger minister could have apologised and stayed. The problem was that the minister was Blunkett, again, brought back into government after a five-month exile. Which goes to show that ministers go when media and political pressures overcome their personal resilience. Blunkett, spirit weakened by pain in his private life and by his previous sacking, went easily. Hunt has put up more of a fight, and has survived the first onslaught.

I am told that, this morning, the Prime Minister will "come out fighting" for him in a long-arranged interview with Andrew Marr on BBC1. No doubt he will deploy any arguments that come to hand. He will point out that laying down the career of a special adviser to save one's own is not new. Stephen Byers did not resign because Jo Moore, his special adviser, wrote that 9/11 was a "very good day to get out anything we want to bury". Gordon Brown did not resign when Damian McBride, one of his special advisers, wrote "juvenile and inappropriate" emails.

As far back as 1954, in the Crichel Down affair, Sir David Maxwell Fyfe, the Home Secretary, laid down the principle that, "where action has been taken by a civil servant of which the minister disapproves and has no prior knowledge, ... there is no obligation on the part of the minister to endorse what he believes to be wrong". That was before Harold Wilson invented special advisers, but the same principle surely applies.

David Cameron may also try the "hand on heart, we all did a bit too much cosying up to Rupert Murdoch" line, which he did in the Commons last week, which is code for "Tony Blair did it". Which he didn't, actually. Everyone, including Hunt last week, goes on about Blair flying "halfway round the world" to pay respects at the court of Murdoch in Australia in 1995, when Blair's speech in fact laid out his disagreement with Murdoch on European policy. And Stephen Byers, when he was Trade and Industry Secretary, blocked Murdoch's bid for Manchester United in 1999, for example. Not that facts matter, because everyone believes that Blair was too close to Murdoch, and it is the show of conviction that counts.

Against that, however, is the question of whether Hunt misled Parliament last week, as this newspaper reports today. Never mind the "quasi-legal" arguments about the BSkyB takeover: that is the kind of simple test that could bring Hunt's career to an end.

twitter.com/JohnRentoul; independent.co.uk/johnrentoul