One of the marks of a good Home Secretary is a healthy wariness of those in authority who come begging for ever-greater powers. The former Labour Home Secretary Charles Clarke failed on that score. Just over a decade ago, the police persuaded him that they needed the freedom to lock up terror suspects for 90 days without trial. The rebels who defeated the Labour plan were right: ten years later no one has presented a case of a suspect who committed an act of terror because they had to be released before the 90 days were up.

Theresa May, who has confronted the police unions with such admirable fortitude, is now showing worrying signs of falling into the same trap as Clarke. This week she published a redrafted Investigatory Powers Bill, after the original one published last year was savaged by a succession of parliamentary committees for its casual expansion of state surveillance powers. The revised bill comes with long documents pleading the case for its two most contentious provisions: the scanning of huge volumes of telecommunications data in order to identify patterns of phone and internet use, and the retention of data on all computer users’ internet activity for 12 months. What powers the security services already possess in the field, claims the first document, have foiled seven terror plots since November 2014.

We can take the Home Secretary’s word for this. But both documents miss the point. The problem with the government’s proposals is not that the powers available to the security services and to those parts of the police which investigate serious crime are too great. By and large, Britain’s spy agencies have earned the right to be trusted: they are not asking for any extra powers and have never abused the powers they have. The problem is that there are insufficient safeguards to prevent these powers being used for trivial purposes. It is not the spooks who need reining in but the wannabe James Bonds of the town halls and local constabularies.

Clearly, if the security services are to tackle terrorism they must be allowed to keep one step ahead of the terrorists, who are well-versed in anti-espionage techniques. Thus the likes of MI5 must have powers to intercept communications and scan what-ever data they need. Snooping it is, but when there are hundreds of lives at stake the invasion of privacy is justified. But, happily, they have always had these powers: the right to intercept anything that can be described as a ‘communication’.

In Canada, separate laws are written: one for the intelligence agencies, and one for everyone else. This is precisely what Mrs May ought to have done. Of the surveillance requests made by various government agencies, just 1 per cent relate to terrorism — and yet the British debate about spying powers seems to be dominated by talk about terrorism. This may make life easier for a Home Secretary trying to get a bill through parliament, but raises grave concerns about the protection of basic British liberties.

The new bill makes pitifully little effort to define when heavy surveillance powers may or may not be used. It merely stipulates that they can be used in the investigation of ‘serious crimes’, without attempting to define what that means. The bill specifically allows surveillance to be undertaken in order to safeguard the ‘economic wellbeing’ of the country — a power which could all too easily be interpreted by tax authorities as justifying the use of heavy-handed surveillance of people who pay their cleaner in cash or involve themselves in other trivial forms of tax-dodging.

Moreover, there is no provision to stop information casually collected during an investigation into serious crime being used for less serious purposes. Police might start off investigating someone for a suspected terror plot, for example, and fail to pick up any evidence of that, but meanwhile discover that he has been watching live TV programmes on his computer without a TV licence. Could police turn him over to TV licensing for prosecution? There is nothing in this bill to say that they cannot.

Without such a safeguard, surveillance becomes almost all-purpose — legally permitted so long as it begins as part of an investigation into terrorism or serious crime. The argument that ‘if you have nothing to hide you have nothing to fear’ fails to address the problem. Surveillance skews law-enforcement towards the trivial end of the criminal spectrum. It is easy, by means of covert cameras, to scoop up huge numbers of people who drive in bus lanes, drop litter, let their dogs off the lead where they are not supposed to. Hand the police records of our internet use and they will also scoop up people who illegally download music, or teenagers who text each other rude photographs. Do we really want law enforcers to spend their time on these things, rather than concentrate on catching the more difficult fish who know how to evade surveillance?

Mrs May will never please the extremists of the privacy lobby, who seem to believe, like the acolytes of Edward Snowden, that all government surveillance is illegitimate. But she would win over most of the rest of us if she took more trouble to define exactly in what circumstances and for what purposes surveillance methods may be used — and when they must not be.