Few would dispute that the Government should work hard to pursue tax dodgers. And in such lean times as these, a crackdown can only be all the more welcome. After all, with nearly £7bn thought to be slipping through the net each year, there is much to be gained for the yawning public purse. But even so large a prize does not give the taxman carte blanche in his methods, and evidence of a sharp spike in snooping by the authorities is still cause for alarm, however worthy the cause.

There is an established legal framework for HM Revenue and Customs' activities. The department can use the Regulation of Investigatory Powers Act to request access to communications data, allowing investigators to track what websites their suspect has visited, where and when mobile phone calls were made or received, and when emails and text messages were sent (although not what was in them).

This is no blanket argument against such powers. Some surveillance activities may be necessary to root out criminal behaviour. As HMRC points out, communications data have helped rake hundreds of millions of pounds into the Exchequer in recent years. All well and good. But too many questions remain unanswered for privacy issues to be dismissed.

As it is, although the extent of HMRC's snooping activities can be established – albeit laboriously – using Freedom of Information Act requests, key details remain elusive. How often are the most intrusive surveillance methods used? How many times have successful prosecutions resulted? Are suspects who turn out to be innocent told that they have been watched? Given the Home Secretary's controversial plans to force internet and mobile phone companies to vastly expand their storage of customers' communications data, such questions are far from academic.

Electronic communications are a tempting trove for investigators. Only absolute transparency can ensure that zealous sleuths do not ride roughshod over the right to privacy.