Watching the cockpit tape that shows the moment when British soldiers came under attack from US planes in Iraq is a terrible thing. You see a military disaster unfolding, step by inexorable step, and you know how it is going to end. For Lance Corporal of Horse Matty Hull, who died, the four men of the Household Cavalry who were severely injured, and for Lance Corporal Hull’s family, who must have found the video agonising, this is a tragedy, played out in real time. But it is a tragedy, too, for the pilots who opened fire on the wrong targets, and who then realised, to their horror, what had happened.

Their language comes across with crystal-clear precision, and it is the language of remorse. You hear four-letter curses, a cry of despair, and the sound of weeping. “I am going to be sick,” says one of them. It sounds as if he was. You do not need to be an expert to spot the mistakes made, both by them and by their headquarters, as the action builds up. Two relatively inexperienced pilots, looking for enemy rocket-launchers, spot a convoy of vehicles close to Iraqi positions, and radio back to base to inquire whether there are any “friendlies” — allied forces — near by.

Their grid references are vague, and no one seems to double-check them. Word comes back that there are no friendlies in the sector they are patrolling, and this too is accepted without question. The pilots can see, from 12,000 feet up, the orange markings that should inform them that these are British armoured vehicles, but they mistake them for enemy rocket-launchers. They go in for the attack with a rush of adrenaline, and later they go in again. It is only when they see, far below them, bodies being hauled away from burning vehicles that they begin to realise what has happened. The message from base — “Abort your mission. It looks like we have a blue-on-blue situation” — strikes a chill, as does the urgent British voice breaking in to add: “Abort, abort.” By then it is too late.

I have no doubt that the coroner, who is holding an inquest into Lance Corporal Hull’s death, was entitled to ask for the tape to be seen in court. It is material evidence and, if not declassified, could certainly be seen in a closed session. To show it in public, however, before it has been cleared by military intelligence, sets a dangerous precedent. Putting out a leaked tape carrying full details of a military engagement in the tense and delicate theatre that is Iraq not only breaks all the rules that govern exchanges of intelligence between British and American forces, it ignores the risk of giving information to the enemy.

Trevor Kavanagh, of The Sun, the newspaper that is running the video on its website, argues that the tape “reveals no secrets, only embarrassing truth”. How can he know? The video contains pilot-to-pilot exchanges, the identification of targets, operational details of the A-10 “tankbuster” communications system and evidence of what it can deliver at 12,000 feet. Whether this is useful to the other side, I do not know. And no more does Kavanagh. The US official, who argued yesterday that, since the video had not been declassified, its premature release broke the “body of law” that governs British-American relations, was right. It may indeed be embarrassing, but it may also put lives at risk.

Who is ultimately to blame for the cavalier way with which military intelligence is dealt these days, and the almost routine way in which it is disregarded? Both governments have been prepared to subvert information in order to further their political ends in Iraq. False dossiers have been compiled and facts bent to support government policy. The Hutton and Butler inquiries showed how Downing Street and the Joint Intelligence Committee had colluded to exaggerate the threat of Iraqi weapons of mass destruction. In Washington, Senate inquiries have been held into how data were falsified in the run-up to the invasion. It is not surprising that, in this climate, the hard and fast rules that once governed the use of intelligence by politicians and the media have become blurred.

A measure of how far that has eroded public confidence can be found in a newly published biography of one of the most remarkable figures in British intelligence — Alastair Denniston, co-founder of the code-breaking centre at Bletchley Park, and the man who was in charge of the Government Code and Cipher School from 1919 to 1942. Almost completely unknown outside intelligence circles, it was he who, with a colleague, went out to Poland in 1939, and brought back the code-breaking data that allowed the British to decipher the secrets of the German Enigma machine.

Thirty Secret Years by Denniston’s son, Robin, shows how the delicate intelligence relationship between Britain and the US was established on two unbreakable principles: free flow of information between allies and absolute secrecy to the outside world. Even mention of the Cipher School was forbidden. Denniston would have been horrified at the notion of negotiating intelligence with government, and even more appalled at the release of information to the media, which could then be combed through by enemy agents.

He, of course, operated during two world wars. But this country is again at war, with British soldiers engaged in battles both in Iraq and Afghanistan against determined enemies. Intelligence is a weapon that needs to be jealously protected, and giving away its secrets or undermining its status may be every bit as dangerous as revealing details of a newly developed rifle, or a missile’s targeting system.

That is a lesson that may need to be learnt all over again, not just by the media, but the Government too.

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