Britain's most senior police officer has been urged to maintain his political independence after he gave his backing to the introduction of identity cards and tighter anti-terrorism laws.

The Metropolitan police commissioner, Sir Ian Blair, said ID cards and a conspiracy law covering "acts preparatory to terrorism" were needed to deal with the threat to Britain from al Qaeda.

Sir Ian's support for such measures would normally be regarded as a regular part of political discussions.

But in the middle of a general election where the issue is so divisive across the major parties, the comments have been branded as "highly political" and "potentially controversial" by civil liberties groups.