A policeman who was given the identity of a dead four-year-old boy said he felt he was "stomping on the grave" of the child.
The officer adopted the fake persona of Pete Black while undercover in anti-racist groups, the Guardian reported.
He told the paper: "A part of me was thinking about how I would feel if someone was taking the names and details of my dead son for something like this."
Another officer, who used the identity of a child car crash victim, said he was conscious the parents would "still be grief-stricken" but argued his actions were for the "greater good".
Both officers worked for the Special Demonstration Squad (SDS), which was apparently disbanded in 2008.