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.
More top news
Thriller writer Anthony Horowitz is at the helm of the latest Bond book, which is based on an unfilmed Ian Fleming TV script.
Actress who plays iconic Daenerys Targaryen admitted she didn't want to become typecast for nudity.
Details of the In/Out referendum will be presented to Parliament in a Bill this morning.