Undercover police officers have reportedly used the identities of dead children and issued fake passports in their names.
According to the Guardian, the Metropolitan Police approved the move for officers infiltrating protest groups without consulting or informing the children's parents.
The newspaper reported that the practice of sifting through national birth and death records in search of suitable matches went on for three decades, with officers issued with official documents such as driving licences and national insurance numbers.
Scotland Yard says the practice is not "currently" authorised, but has announced an investigation into "past arrangements for undercover identities used by SDS (Special Demonstration Squad) officers".
One officer told the Guardian that he felt he was "stomping on the grave" of the four-year-old boy whose identity he used, while working undercover in anti-racist groups.
"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," he said.
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 could be justified because they were for the "greater good".
A Scotland Yard spokesman said: "A formal complaint has been received which is being investigated by the DPS (Directorate for Professional Standards) and we appreciate the concerns that have been raised.
"The DPS inquiry is taking place in conjunction with Operation Herne's investigation into the wider issue of past arrangements for undercover identities used by SDS officers.
"We can confirm that the practice referred to in the complaint is not something that would currently be authorised in the MPS (Metropolitan Police Service).