When three former home secretaries sign a public letter on a matter of national security, the intention is that they be taken seriously.

When the trio are senior members of the opposition, we are also supposed to understand they have no party political interest in the matter. They act for the greater good.

When it comes to terrorism and the like, Jack Straw, Alan Johnson and David Blunkett share a common expertise. Each has worked closely with the security services and can presumably claim to understand a thing or two. But each also has a knack for the "if only you knew" gambit.

They have a counterpart in General Keith Alexander, head not only of the United States' National Security Agency but of what is called – no melodrama, you understand – Cyber Command. This week, with the Obama administration besieged by accusations that it has presided over the mass surveillance of innocent citizens, the general stepped up to tell Congress that "dozens" of terrorist plots have been thwarted thanks to spying by the state.

He didn't bother to name a couple, of course, for the sake of public enlightenment. Messrs Straw, Johnson and Blunkett, like most British ministers past and present, are guilty of the same sort of reticence. Dozens of plots, hundreds of plots, even thousands: the vague numbers alter, but the claims are substantiated only when cases come to trial. Trials happen rarely.

It may be that all these people are telling the absolute truth. It's entirely possible that the things they have seen and heard would make your hair stand on end. But all they ever offer publicly is the equivalent of "trust us". After The Guardian's revelations over the US's Prism programme, and the confirmation that Britain's GCHQ is little better than an NSA franchise, this isn't quite good enough.

Undaunted, our three former home secretaries add their names – along with the Tory peers King and Baker, and the LibDem Lord Carlile – to a letter in The Times demanding that Nick Clegg should get out of the way and allow a revived communications data bill. That would give Britain's security services an effective Prism programme of their own. Theresa May, the present home secretary, backed by David Cameron, wants it very badly.

The first draft bill was dropped from the Queen's Speech because of Liberal Democrat objections. That should have been the end of it, but in these matters the Home Office never rests. The killing of Drummer Lee Rigby – by a pair who were already known to the security services irrespective of internet use – has inspired the security establishment to try again. Judging by the intervention of Messrs Straw, Johnson and Blunkett, Labour seems prepared to help out.

The letter asserts that "Coalition niceties must not get in the way of giving our security services the capabilities they need to stay one step ahead of those that seek to destroy our society". Though there is no evidence whatever that mass surveillance would have saved Drummer Rigby, the authors use the horror of his death to claim that "when such a threat reveals itself, government has a duty to ensure they [the security services] can do all they can to counter it".

As things stand, police can pin down who has made a phone call or sent a text message, when they did so and where they were located. Now government wants the rest: internet browsing, emails, instant messaging, social media, Skype, even internet gaming. They also want this information, gleaned from every last one of us, to be stored for a year, just in case.

As ever, their political representatives offer the smooth assurance that "if you've done nothing wrong you have nothing to fear". The idea that the wholly innocent might have rights is not examined. What they propose is equivalent to placing a whole town in detention on the off-chance one person decides to rob a bank.

The state doesn't want actually to open your emails – not without a warrant, at any rate – and for some reason this is supposed to be reassuring. What the spies do want to know is every contact, the "who, when, where and how". From this a complete picture can be constructed of your every act and habit. How long in the pub? How much time spent in the company of someone who might not be your spouse? In the jargon of old cop shows, they want to put a trace on every citizen.

Think about it for a second. We have a threat that is real enough: so much we know. How big is the threat? Home secretaries past and present can only manage "if only you knew". So would this be emergency legislation, liable to be repealed if and when the threat fades? I have not found a single advocate of the bill making any such promise. For the moment, the spectre is terrorism. Next month it could be paedophiles or drug dealers. Ms May has used those examples often.

Last October, she told the joint parliamentary committee examining her first attempt at legislation that police had made "urgent requests" for communications data in 30,000 cases in 2011 (from an annual total exceeding half a million requests). Then the home secretary made an extraordinary claim. She said: "Of the 30,000 estimated cases last year - between 25% and 40% of them resulted in lives being saved." That's at least 7500 lives. And this is a person we are supposed to trust?

Messrs Straw, Johnson and Blunkett were all keen on ID cards, once upon a time, and on 90-day detention, and on almost every other set of new powers "essential" to the eternal battle against terrorism and other crimes. Until now, enough has never been enough. In whatever form it takes finally, the communications data bill brings the dream of comprehensive, continual surveillance to fruition.

Like Barack Obama, these politicians tend to say sagely that the right to privacy must be "balanced" against the need for security, as though one can only gained if the other is lost. That's specious logic. But by no accident it overlooks a detail common to every state security system in history. Has a spook ever once said "we think we have too much power; we'd like to give some back"?

There are reports from MPs that a host of agencies are agitating for access to the data trove. HMRC is already in on the act. Local authorities, with some power to access data under the Regulation of Investigatory Powers Act, are said to be asking for more in the existential struggle against flytippers and folk who put the wrong things in their bins. When spying becomes institutionalised, everyone with a bit of power wants a share.

Blithely, the politicians use our foolish habits against us. Doesn't "everyone" put "everything" on Facebook and reveal their every thought on Twitter? When we give so much away, what's the harm in the state bringing a little order to collection and collation? Choice, the thing that defines liberty, might have something to do with it.

Meanwhile, terrorists and smart crooks will in future know exactly which electronic gadgets and services to avoid. The rest of us will be criminalised by default.