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.
More top news
A member of the Dutch air force has travelled to Syria and has probably joined the Islamic State group, the Defence Ministry said.
Chaotic scenes in Hungary continued throughout the day with many migrants refusing to leave a train.
Prosecutors will seek the death penalty for a man accused of shooting dead nine people at a church in South Carolina.