Idiots. Utter, unbelievable, jaw-dropping, unpardonable idiots. It is beyond farce, past comprehension, criminally irresponsible and beneath contempt.

All those lectures from government and authorities about keeping our personal data safe; every statement ever made about the security of the proposed NHS database of everybody's personal medical records; each claim that the Children's Database containing all their personal details will somehow make our kids safer; and of course each and every promise about the safety of the national identity register — exposed as quite, quite worthless. Because as soon as you put it on a computer, a bloke in an office can download it and stick it in an envelope and send your most personal details and mine and our children's across the country with a dodgy courier.

It is shocking, it is risible, it is hilarious. Someone gave a disc containing confidential data about 25 million people to a bloke on a bike? And he lost it? Of course, a case of mass identity or financial fraud would never happen in this way. It is too chaotic. Fraud will happen through a far more organised infiltration of the official systems; but what yesterday's revelation does is underscore the insecurity of those systems. And allows us to giggle at the po-faced pretence of those in authority that they are any better at protecting us than we are ourselves.

This is the pretence at the heart of every state attempt to tighten up national security — through searches and ID cards and barricades and banning water in airports and making us take our shoes off. All these measures put the public to ever-greater inconvenience while it knows that terrorism happens through random and unimaginable acts that no amount of searching and barricading can block.

Likewise, it is the very randomness of the loss of data that shocks. Someone just did something you couldn't have predicted: he stuck a load of incredibly sensitive stuff about us in the post. And it was (almost certainly) randomly lost. It's probably in a rubbish dump somewhere by now.

It might have been random, but it betrays a total and arrogant carelessness about the privacy of the individual. And it wasn't just one guy; it happens often. It was clear from Alistair Darling's statement to the Commons yesterday that there is systemic security failure at Her Majesty's Revenue & Customs.

It isn't the first time recently that the organisation has lost personal data. Turns out HMRC routinely sends sensitive information around the country on discs. Earlier this month the details of more than 15,000 Standard Life customers, including pensions, were put on a disc and lost by a courier en route from HMRC in Newcastle to the Standard Life HQ in Edinburgh. Last month a laptop with data about 400 people with high-value Isas was stolen from the boot of a car belonging to someone at HMRC. Personal and financial details have been misdirected to wrong addresses or found in the street.

Mr Darling looked shaky in the Commons, as well he might: first shaken by Northern Rock and now drowning in a flood of misplaced personal information. The Government's entire public IT agenda — all those systems and databases and supposed safeguards — is now under threat. His statement was fine and comprehensive, but it became risible at one point: when he claimed that ID cards would somehow have made this lost information safer because we would only have been able to access it with biometric identification. Yeah, us and every employee at HMRC and any other official busybody, just as our personal medical details are to be made available to any passing temporary employee in the local A&E.

This will be a test of Gordon Brown. His Government is at its best in a crisis. The series of problems over the summer — bombs, floods and foot-and-mouth — usefully stamped his authority on the country and gave his administration the impression of action and progress. They hid his lack of a plan. But those problems were harder to lay directly at the foot of a government agency, for which ministers indisputably have responsibility - and, in this particular case, for which the Prime Minister himself had responsibility for ten years until June. He was right to turn up and sit next to Mr Darling in the Commons yesterday.

Mr Brown is getting a reputation even among his closest colleagues for bullying and blaming others when things go wrong, as they did in the on-off election fiasco. Things are not going well in No 10, with even some of the Prime Minister's closest allies questioning the Brown project. Mr Brown's friends - yes, friends - talk of rages and impregnable sulks.

He governs by small inner circle — issuing sudden edicts to otherwise paralysed government departments — yet he has dangerously few diehard, close friends left. With the uncertain start, officials wonder what he spent the past ten years planning. A power battle is already shaping up for the succession, with paranoid allies of the Prime Minister, and supporters of future leadership contender Ed Balls, publicly slapping down the other young pretender David Miliband. A scramble for the succession! And he has been in office for less than five months.

So how he handles this fiasco at HMRC — whom he supports and whom he blames — will be a critical test. His Chancellor was already weakened; damaged by Northern Rock and perceived, within the Treasury, as neutered by No 10. Mr Darling, remember, considered giving up politics seven years ago to spend more time with his family, confiding to a journalist: “I don't see politics as a career.” The Prime Minister had better stand shoulder to shoulder with him now, and share the fallout; there is a lot more at risk than a missing disc.

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