Press law - So why did he do it?
Why did David Cameron take his own deputy as well as some senior figures in his own party by surprise by pulling the plug on all-party talks to create a new press regulator?
Just three hours before the prime minister declared that the talking was over and it was time for MPs to choose between the Tory or the Labour/Lib Dem approach, Nick Clegg had told the BBC: "We had a productive meeting - David Cameron, Ed Miliband and myself - yesterday afternoon. We'll have further discussions and I hope we'll make progress… I'm not going to start imposing artificial timetables"
Cameron's critics will say that the answer is simple: he has chosen to side with the press barons - the Murdochs, Barclays, Rothermeres and co - and to ignore the victims of press hacking and intrusion - the Dowlers and the McCanns.
When I put this to the prime minister he answered, in effect, that what was good for the victims was to have a workable solution ie a system of press regulation which is tough but with which the newspapers will co-operate, and to have it set up soon.
Whoever is right a more subtle political calculation has been made inside Number 10.
The PM has always known that he is likely to be defeated in the Commons on this issue. It is Ed Miliband and not him who leads a coalition on press regulation of Labour, the Lib Dems and some Conservative MPs and peers too.
They had found a way to tack votes on a new press regulator onto other government bills setting up, for example, a National Crime Agency or a new law on defamation. Government business managers would have faced the choice between losing those measures or keeping them with a press law attached. When they did so Cameron would have been accused of being in office but not in power- someone who couldn't, to quote the Labour leader, organise anything in a brewery.
So Cameron decided to pre-empt them and to pick a fight on his own terms and at a time and place of his choosing. By doing so he still faces the likelihood of defeat (though nothing is certain on an issue on which few have fixed views). He will still face being attacked for abandoning the victims.
However, he will calculate that he has demonstrated to the public that he is willing to deal with the issue of press excesses at the same time as indicating to the newspapers that he is fighting for press freedom whilst his opponents are desperate to shackle the press. He will argue that the public want parliament to spend time on the economy, welfare reform, schools and the NHS whilst his opponents want to get bogged down on the minutiae of press regulation.
Above all though, perhaps David Cameron's calculation was that his defiant Downing Street news conference would make him look like a leader at a time when his own party has been questioning how long he should carry on in the job.