Princess-in-waiting: Woolworths has already designed a souvenir plate in anticipation of William and Kate’s wedding

Does Prince William’s girlfriend Kate Middleton ever have a bad hair day? We know, thanks to Tatler magazine, that she blow-dries. But she had better keep some spare batteries. For straggly locks will no doubt be regarded as world news.

Tuesday’s Evening Standard pleaded “Charles must protect Kate”, above an enormous photo of her looking rather upset. Protect from whom? Next week if the dryer dies, the headline could be “Heir must protect hair”.

The Standard is not alone: everyone is at it. The most blatant piece of self-justification came from the head of the Big Picture agency, who denied that sending huge blokes to poke lenses in a girl’s face on her way to work was intrusion. “We took Kate’s picture because it’s a news picture,” he blustered. “She’s in the news. If she wasn’t, she wouldn’t be in the least bit interesting.”

Right. But why was she in the news? Oh yes. Hacks had wrested all the coverage they could out of her parking ticket (that Diana, Princess of Wales, also received a parking ticket before her engagement is a parallel that will surely keep historians enthralled). So one of them decided that she must be about to get engaged to Prince William. The paltry amount of evidence for this does not seem to have mattered. For every canny editor knows that a sufficiently intense and hysterical bout of speculation can drive events. And at the very least it allows programmes and papers to fill time and space cheaply by reporting feverishly on each other’s reports. I have no doubt that some TV producer will soon have one pundit interviewing another about whether Ms Middleton might have a bad hair day, when it will strike, what it might look like and what it might say about her ancestry.

This royal saga may seem like frivolous fun. But speculation is a media disease that has serious consequences. As a preference for guesswork over facts takes hold in more and more parts of the media, it weakens the credibility of all of it. Fewer and fewer people are going to trust journalists who present innuendo and gossip as fact. Fewer and fewer people will feel like defending the liberty of a press that takes so many liberties. And there are plenty of dubious characters who would like stronger privacy laws to protect them from the scrutiny that the press should provide.

The “serious” media are not immune. Only last month the BBC set off a rush to reproduce every single picture, rumour and piece of gossip available about first one then two men arrested in connection with the murder of the Ipswich prostitutes. Neither broadcasters nor newspapers seemed remotely concerned that they could be hampering the course of justice, despite their endless ullulating about the horror of the killings. What has it done to Tom Stephens to be branded a “loner” who knew all the girls, pictured exclusively in fuzzy weird-looking photos? Now that he is no longer helping the police with their inquiries, what will readers and viewers conclude about the media they rely on?

Some speculation is dangerous, almost all is pointless. “How do you think you’ll do in the next game?” is now a stock question in sports interviews, which usually gets the answer it deserves. “The lads are ready to do their best,” says the hapless football manager. Most people understand that they can’t predict the future and that they’d look damn silly trying; only pundits and TV producers think otherwise. For they know two important things: first, that no one will remember their predictions so they can remain blissfully unaccountable — and make money. Second, speculation is cheap. No research is needed, no investigative teams, no reels of footage requiring painstaking editing. Hence the rise and rise of formats in which one journalist interviews another about something that neither of them knows about or can know about, because it has not yet happened. The accession and policies of Gordon Brown are a favourite, despite most senior commentators repeatedly stating that they don’t really understand him.

Does all this matter? Yes. Not only because endless speculation makes everything so superficial, but also because it compels crises. A live discussion on “whither farming?” tends not to get producers as excited as one called “the imminent collapse of British food”. If we journalists spent as much time examining the outcome of policies as we do speculating about whether there will be a Cabinet reshuffle, who will get what jobs and what they might do, we would provide a better service.

Facts don’t have to be dull. The recent news that the police have lost track of 500 potentially dangerous offenders was anything but. Note that this story was not uncovered by a journalist. Nor was last year’s astonishing revelation that the Home Office had lost almost 1,000 foreign prisoners: that was exposed by a dogged backbench MP. Remember the row about top public schools colluding on fees? That was prompted by two pupils who hacked into their school computer. There are still investigative reporters digging out the truth. But guesswork infects some of the most basic news stories. Yesterday I switched on Radio 4 to hear that “hundreds” of people may have been affected by loan scams — “or thousands”. Well which is it? This kind of speculation drives hysteria.

I love a royal soap as much as anyone. But a good soap has a solid plot. Until there is an engagement, there isn’t one. The decision of News International — owner of The Times — not to publish any more snatched pictures of Kate Middleton has dampened things down for now. But the temptation to speculate will not go away, on this or any other issue. Journalists need to rein that temptation in, otherwise we may find that fewer and fewer people can be bothered with us any more.