The former Director of Public Prosecutions said police appeared to have "completely lost their moral compass" after a report emerged about officers using the IDs of dead children.
Lord Macdonald, who has called for a pubic inquiry, told the BBC Radio 4 Today programme:
How are you supposed to maintain a level of fair and objective evidence-gathering if you are ... using a dead child's identity?
We don't know quite how these units were operating in days gone by. It looks as though they've effectively gone rogue.
What we really need is a public inquiry into undercover policing. This is a deeply ethical issue which the police have to grapple with.
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