The second data protection principle is that—
the law enforcement purpose for which personal data is collected (whether from the data subject or otherwise) must be specified, explicit and legitimate, and
personal data so collected must not be processed by or on behalf of a controller in a manner that is incompatible with the purpose for which the controller collected it.
Paragraph (b) of the second data protection principle is subject to subsections (3) and (4).
Personal data collected for a law enforcement purpose may be processed for any other law enforcement purpose (whether by the controller that collected the data or by another controller) provided that—
the controller is authorised by law to process the data for the other purpose, and
the processing is necessary and proportionate to that other purpose.
Personal data collected for any of the law enforcement purposes may not be processed for a purpose that is not a law enforcement purpose unless the processing is authorised by law.