Rebekah Brooks.

What can you say? Well, not a lot, because any comment on the case of the horse-loving former editor of The Sun, who was arrested at dawn on Tuesday on suspicion of conspiracy to pervert the course of justice, would prejudice her trial. But since her involvement in the phone-hacking scandal comes up every day in the Leveson Inquiry, I'm not sure whether the normal sub judice rules apply. Lord Leveson is now conducting a running commentary on a police investigation and throwing up new suspects by the week.

The police are having to run fast just to keep up with the arrests. And since the suspects list includes the police themselves, following the allegations by Deputy Assistant Commissioner Sue Akers about a "culture of illegal payments" to the police, the whole thing is in danger of turning into a uniformed farce. "Ok, Sunshine – I'm nicked!" But this is a serious business, at least for anyone who still believes in the values of a free press.

The Leveson Inquiry has effectively placed the entire newspaper industry in the dock, and all but declared it guilty as charged. Guilty of lies, deceit, theft, stalking, phone hacking, corrupting the police, invading peoples' privacy, breaking the law with impunity on everything from the data protection acts to criminal trespass. This is not a problem confined to the Currant Bun. Newspaper executives across Fleet Street are fingering their collars nervously this week, desperately trying to remember what they may or may not have said about such and such a story back in the day. Waiting for the doorbell to ring at 5am because we know from the 2006 report from the UK Information Commissioner Richard Thomas that they were all "at it". That extraordinary report, "What Price Privacy", exposed an "extensive illegal trade in confidential personal information". One private detective alone had conducted thousands of investigations for 305 named journalists across all UK national papers.

Why didn't the police stamp this illegal trade? Well, if Ms Akers is to be believed, many in the Metropolitan Police were up to their necks in it themselves. We always knew that the police were hand in glove with the press, but we didn't know they were hand in pocket. In future, the police will probably have to register every contact with a journalist and every off the record briefing – which of course, makes it on the record. This is where it gets serious because it is in the nature of journalism that the most important stories generally emerge from confidential information.

Lord Leveson has talked of setting up an "independent body backed up by statute" to police the press. This watchdog would have powers to register journalists, uphold codes of conduct and impose fines on miscreants. It wouldn't necessarily put the press under political direction but it would certainly make the responsible journalists work much harder.

Regulatory bodies love compliance, a bureaucratic and time-consuming process of demonstrating that the rules are being obeyed. Editors may have to minute every conversation about any story that might involve breaking the rules. They might even have to seek prior consent from the regulator, probably called Ofpress, before an investigation is undertaken to ensure any laws are being broken in the public interest and not in the interest of promoting prurient gossip.

It could make it very hard to break stories, such as The Herald's disclosure of the Scottish Criminal Cases Review Commission's report on the withholding of evidence in the Megrahi case. Lucy Adams's exclusive story perhaps technically broke the data protection laws, even though the public had a right to know. Similarly, the MPs' expenses scandal only emerged because the Daily Telegraph in 2010 paid an employee of the Palace of Westminster £110,000 for a computer disk. This was arguably bribery of an official, invasion of privacy, breach of the data protection acts and many other things. It was also thoroughly justified in the public interest.

It will be said by people who know nothing about the practice of journalism that, well, other bodies have to be accountable, so why should the press be any different? It's a difficult one to answer and so far the press has been lamentably poor at articulating an answer. The problem is that as soon as you start making the case for a free press, it sounds as if you are condoning the methods of The Sun and other tabloids. But we can't allow the delinquent behaviour of the few to become a pretext for extinguishing freedom of speech and freedom of information for the many.

Especially here in Scotland, where the courts have taken a rather more liberal approach to press disclosure in the past. Scottish papers, such as The Sunday Herald, have been able to breach super-injunctions concealing the identities of celebrities such as Ryan Giggs. In 1988 an injunction preventing the publication of the book Spycatcher, about the activities of the secret service, was broken by the Scottish press. If a UK-wide watchdog body is set up with statutory powers regulating disclosure, that might have a chilling effect on the courts. Fortunately, the Scottish Parliament will have a say on any legislation that sets up the new press regulator, and let's hope our legislators have their wits about them. Unlike broadcasting, the press is not reserved to Westminster – though I'm not sure Lord Leveson realises this.

The trouble with putting judges in charge of the press is that they don't understand how it works. Lord Leveson demonstrated this when he said that social media sites like Twitter don't need to be regulated because they are "like people chatting in a pub". He doesn't realise that when broadcast over the internet this chat becomes public information, it becomes news, just as if it had been reported in a newspaper. This was effectively how the recent super-injunctions taken out by celebrities such as Andrew Marr were undermined. We had the ridiculous situation where, week after week, the press could not report on matters that were common knowledge on the internet.

The danger is that legitimate investigative journalism will become almost impossible because of statutory legislation, while anything posted willy nilly on social media sites is effectively exempted from the same repressive laws.

Lord Leveson seems about to introduce a system of regulation just at the moment when such regulation has become an anachronism. The stable door is closing just as the horse has bolted – with Rebekah Brooks clinging to the saddle.