But suppose that our journalist takes out the word "rumour" and substitutes "YouTube" or "blogger" as the source. Then, going by recent experience, editors will nod it through, possibly commending their man or woman for judicious use of the internet. The BBC and other television stations happily run nightly pictures of mayhem from Syria, grandly disclaiming responsibility for their authenticity. These disclaimers are intoned so often that they now have as much impact on viewers as warnings that a news report may contain flash photography. People understandably believe that if the BBC and other channels were not convinced of the truth of YouTube pictures they would not be using them as their main source of information on Syria.

YouTube pictures may have played a positive role in the uprisings of the Arab Spring, but the international media is largely mute about how easy it is to manipulate them. Pictured from the right angle, a small demonstration can be made to look like a gathering of tens of thousands. Shootings in one street in one town can be used to manufacture "evidence" of shooting in a dozen towns. Demonstrations need not be genuine events luckily captured on mobile phone cameras by concerned citizens; frequently the only reason for the protest is to provide material for YouTube. Television companies are not going to reject or underline the stage management of film that is free, dramatic, up to date – and which they could not match with regular correspondents and film crews even if they spent a lot of money.

In the print press, bloggers get an equally easy ride, even though there is no proof that they know anything about what is going on. Hence the ease with which a male American student in Scotland was able to pretend to be a persecuted lesbian in Damascus. Since the Iraq war, even the most intensely partisan bloggers have been presented as sources of objective information. Tarnished though they may now be, they still have a certain cachet and credibility.

Governments that exclude foreign journalists at times of crisis such as Iran and (until the last week) Syria, create a vacuum of information easily filled by their enemies. These are far better equipped to provide their own version of events than they used to be before the development of mobile phones, satellite television and the internet. State monopolies of information can no longer be maintained. But simply because the opposition to the Syrian and Iranian governments have taken over the news agenda does not mean that what they say is true.

Early last year I met some Iranian stringers for Western publications in Tehran whose press credentials had been temporarily suspended by the authorities. I said this must be frustrating them, but they replied that even if they could file stories – saying nothing much was happening – they would not be believed by their editors. These had been convinced by exile groups, using blogs and carefully selected YouTube footage, that Tehran was visibly seething with discontent. If the local reporters said that this was a gross exaggeration, their employers would suspect that had been intimidated or bought off by Iranian security.

There is nothing wrong or surprising in revolutionary movements engaging in black propaganda. They always did so in the past and it would be amazing if they did not do so today. My father, Claud Cockburn, fighting on the government side in the Spanish Civil War, once fabricated an account of a revolt against General Franco's supporters in Tetuán in Spanish Morocco. He was bemused when in later years when he was furiously criticised for what he deemed was a neat propaganda coup, as if disinformation had not been a weapon used by every political movement since Pericles.

Such ploys have not been made obsolete by advances in information technology in the past 20 years. These are usually portrayed as being a wholly benign and democratic development that inspired the uprisings of the Arab Spring. And so, to a degree, it was. The iron grip of police states over the media and all other sources of information was broken across the whole Middle East. Governments discovered that the crude repression of the past could be counter-effective. In Hama in central Syria in 1982, President Hafez al-Assad's forces killed an estimated 10,000 people and destroyed the Sunni rebellion but there was not one picture of a corpse. Today scenes from such a massacre would be on every television screen in the world.

So technical advances have made it more difficult for governments to hide repression. But these developments have also made the work of the propagandist easier. Of course, people who run newspapers and radio and television stations are not fools. They know the dubious nature of much of the information they are conveying. The political elite in Washington and Europe was divided for and against the US invasion of Iraq, making it easier for individual journalists to dissent. But today there is an overwhelming consensus in the foreign media that the rebels are right and existing governments wrong. For institutions such as the BBC, highly unbalanced coverage becomes acceptable.

Sadly, al-Jazeera, which has done so much to shatter state control of information in the Middle East since it was set up in 1996, has become the uncritical propaganda arm of the Libyan and Syrian rebels.

The Syrian opposition needs to give the impression that its insurrection is closer to success than it really is. The Syrian government has failed to crush the protesters, but they, in turn, are a long way from overthrowing it. The exiled leadership wants Western military intervention in its favour as happened in Libya, although conditions are very different.

The purpose of manipulating the media coverage is to persuade the West and its Arab allies that conditions in Syria are approaching the point when they can repeat their success in Libya. Hence the fog of disinformation pumped out through the internet.