The police rely on invading citizens' privacy, a senior police officer told a human rights conference on Saturday.
But Roger Gaspar, deputy director general of the National Criminal Intelligence Service (NCIS), said such invasions have to be proportionate to the threat.
"If you don't want privacy invaded at all, that's the end of the police service," Gaspar said. "We need some sensible debate on the limits of our invasion of your privacy."
Gaspar, who was addressing a conference organised by human rights campaign group Liberty, argued that technological surveillance was vital for fighting crime in a highly mobile population.
Traditional community policing techniques, where officers knew those in their patch, were inadequate in an era of easy, unrestricted movement across Europe.
"Yes, we do bug places. We do tap phones. We do intercept email. I don't believe we do this for anybody who is not a serious criminal," Gaspar said.
"We do no interception of minor cases, or even medium cases," he added. "You've got to be in the top one per cent of all criminals."
Gaspar was questioned by Caspar Bowden, director of IT policy think-tank the Foundation for Information Policy Research, about the NCIS policy for accessing traffic data.
This excludes the content of messages, but for email includes subject lines, addressee and time of sending. For a phone call it includes the number dialled, time and length of the call, and for mobiles, the approximate location of the caller.
Press reports have suggested the police would like to force retention for seven years. Gaspar replied that NCIS has never asked for email content to be retained.
In conversation later, he added that the police already has access to some traffic data - it is stored by phone companies to allow billing, in many cases for six months - and just wants to standardise the period of retention.
"We want a debate on this period," he said.
However, deputy information commissioner David Smith made it clear that his organisation will fight NCIS on this issue.
"The police are pressing for information to be retained by your internet service provider and your phone provider, which isn't needed for any purpose except for use by the police," he told the audience. "We are resisting that."
Data protection law states that information should only be retained for the use it is given. It is legal for a phone company to hold traffic data in order to calculate a bill and deal with any customer queries, but it must delete it afterwards.
With an appropriate warrant, the police can see the data - if it still exists.
Smith also said that he had doubts about some aspects of current legislation.
"The Data Protection Act, in our view, does have its limitations," he said. "Some areas are over-bureaucratic."
It also lacked powers to hit those who flouted it, he added.
Liberty is planning to campaign for a privacy act which would make some invasions of privacy illegal.
Law in this area is currently split between several offences, despite article eight of the European Convention on Human Rights - part of English law since last October - offering everyone "the right to respect for his private and family life, his home and his correspondence".
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