Members of the protest  group 'Avaaz'  campaign outside the Houses of Parliament for the government to implement legislation following the Leveson Report.

Members of the protest group 'Avaaz' campaign outside the Houses of Parliament for the government to implement legislation following the Leveson Report. Photo: Oli Scarff

You know I don't want to be more at odds than usual with public opinion; but I have just read the Leveson Report - all four volumes of horror - and my first reaction is that the British press is really rather magnificent. Every paper, virtually without exception, can claim to be running at least one good and original campaign against some abuse, and some of them run several at once.

One thinks of the Daily Mail and its historic, brave and ultimately vindicated decision to name the killers of Stephen Lawrence. Or there is the good old Guardian, exposing everything from the dumping of toxic waste in Africa to the mistreatment of prisoners. The Times has discovered a rich seam in the tax dodges of zillionaires.

The Sunday Telegraph has defied the censors of the Ministry of Defence to expose the ludicrous and shameful treatment of a British sergeant who was sentenced to 18 months' detention for receiving a Glock pistol as a present from Iraqi soldiers he had helped to train.

Even the News of the World (RIP) had some illustrious battle honours. How would the scandal of the cricket match-fixing have come to light if it hadn't been for the derring-do of Mazher Mahmood, the fake sheikh? Every day the British press is out there doing its best to root out the stitch-ups, cover-ups and conspiracies against the interests of its readers. They have helped to keep British politics as free from corruption as anywhere on earth.

So it is all the more miserable to read the shortcomings that Lord Justice Leveson has rightly identified. Indeed, reading parts of his report is like being immersed in a slurry lagoon. The press have bullied and bugged and lied and smeared and cheated. They have shamelessly traduced the reputations of people who turned out to be wholly innocent - such as Christopher Jefferies, or the parents of Madeleine McCann - and then offered only the briefest and most contemptuous of apologies. They have shoved their slavering snouts into the parlours of weeping widows, and by their outrageous lies they have driven the relatives of their victims to suicide.

When you read Leveson in full, you are left to ponder the mystery of how people can behave like this. Are these journalists that much nastier and more cynical than the rest of the human race? Why do they seem to have got out of control? The answer is simple. The press are no nastier than anyone else; quite the reverse. On the whole, journalists are highly intelligent, amusing and frequently idealistic.

But these days they are afraid. They are like the crew of a plane whose port engine has failed. They can see the ridge of the mountain ahead, and they have been driven to start chucking their principles overboard in the hope of avoiding a crash. That is why they have so hungrily hacked phones, bribed officials, and claimed that asylum seekers have eaten the donkeys of Greenwich Park.

They can see the altimeter of their circulation figures spiralling downwards, and they need stories ever more exotic and titillating to keep the readers buying. Newspapers have always chased exclusives. But the pressure on circulation is now so great some papers have abandoned their grip on ethics and reality. In the past 10 years, virtually every paper has experienced a drop in hard-copy circulation of between 30 and 40 per cent, and in some cases more.

People are getting their news from different sources - principally the profusion of electronic media - and there seems to be no stopping the erosion in support for traditional papers. That is in itself a sad and bad thing, since it is the Fleet Street papers that have the skills and experience - and public trust - to expose wrongdoing. But for some papers the costs are becoming prohibitive. Every year, every month, they are losing ground to blogs and Twitter and Google News; every year the internet eats more destructively into the business case for old-fashioned journalism. That is at least one of the reasons why some journalists are behaving so disgracefully, squawking ever louder, no matter how erroneously, in the hope of being noticed.

The tragedy is that the cure may now be worse than the disease.

Leveson is proposing to throw shackles around that part of the media that is already struggling - while doing nothing to tackle the riot of bile and slander on the web. It was Twitter that turned the BBC's awful Newsnight into a monstrous libel of Lord McAlpine; and yet Leveson proposes no code of conduct for the tweeters. Instead, he endorses just about every politically correct criticism of the mainstream press, to the point where he seems to want to sterilise it of fun and flavour.

He complains, for instance, that the Mail was wrong to say that an asylum seeker was given leave to remain because of the attachment he had formed to his cat. I read the judgment, and the cat was certainly mentioned. It struck me as an entirely legitimate headline. He seems to want to make the British press as earnest as the Neue Zurcher Zeitung, whose front-page splash was once about ''100 years of Electric Light in Switzerland''. He wants above all to create a system whereby newspapers would be compulsorily regulated by law - licensed - for the first time since the 17th century. He must be resisted; and the only way is for the media to recognise humbly and sincerely the extent of their recent errors, and accede to the setting up of a powerful and independent monitoring body, capable of wielding hefty fines, whose code they would undertake contractually to obey.

David Cameron's analysis last week was entirely correct. He has thrown the papers a lifeline, and they need to grab it tight. We want a vital and exuberant media that reports the foibles of the rulers, without fear or favour but also without lying and cheating and cruelty.

If the papers get it right, and act fast, they can rebuild trust, and they may also be able to rebuild some of that lost circulation.

Boris Johnson is mayor of London.