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
The Interview, the Sony film about a fictional plot to assassinate North Korean leader Kim Jong Un, has made more than $15m in online sales.
Jamie Oliver turned down the offer of playing a hobbit in the movie adaptation of the book by JRR Tolkien, the TV chef has revealed.
Children are being put at risk as 11% of posts for permanent children's social workers remain unfilled, experts have warned.