The government is expected to publish guidelines this week about how much information ISPs will have to make available to police about their customers. The controversial measures were granted by Home Secretary David Blunkett in a bid to counter terrorism.

The measures, announced on 15 October, gave the police permission to track the online movements of any web user as part of their investigation into a crime. Such increased powers were deemed necessary following the 11 September terrorist attacks in the US, in which email was implicated as a means by which the terrorists co-ordinated their movements.

One key aspect of the provisional legislation is the extension of the length of time ISPs are able keep records on their customers, which track what they are doing online.

Under current data protection laws companies have to destroy logs of emails sent by customers after as little as three months. But the Home Office now wants this data to be kept permanently in case it is needed for future terrorism investigations.

Ispa (Internet Service Providers Association) has expressed its concerns about data retention and has told the government it wants clear information on what data will need to be kept and for how long.

"Ispa welcomes a code of practice that is reasonable and practical in its requirements and effective in its objective," the organisation said in a statement.

The Home Office could not give details on what decisions had been reached at this date, but said it was aware of security concerns and people's unease at intrusive powers.

Industry sources fear the proposals will mean web users' every move could be tracked and perhaps used against them, and a likely concern of all internet users.

The Home Office will have a tough job on its hands if it is to cull people's fears over invasion of privacy and at the same time help the police beat terrorism.