Some of the minors tasked with gathering intelligence are under 16, the report by a House of Lords review committee said.
The report, published last week, was produced in response to proposals brought forward by government officials to extend the amount of time a person under the age of 18 may be used as a covert human intelligence source (CHIS) from one month to four.
'Risks to welfare'
Lord Trefgarne, chairman of the Secondary Legislation Scrutiny Committee, said the policy of using young people as a CHIS had caused "considerable anxiety" among committee members, and he expressed concern about relaxing current restrictions.
"We are concerned that enabling a young person to participate in covert activity for an extended period of time may expose them to increased risks to their mental and physical welfare," he said.
The report, released on July 12, does not specify how many people under 18 have been deployed as informants.
The Home Office, the ministry responsible for law and order within the UK, said covert operatives under 18 may have unique access to information, particularly with criminal gangs, but are used "in very small numbers".
"Given that young people are increasingly involved, both as perpetrators and victims, in serious crimes including terrorism, gang violence, county lines drugs offences and child sexual exploitation, there is increasing scope for juvenile CHIS to assist in both preventing and prosecuting such offences," Ben Wallace, a Home Office official, said in response to a request for clarifications from the committee.
"Much as investigators would wish to avoid the use of young people in such a role, it is possible that a carefully managed deployment of a young person could contribute to detecting crime and preventing offending."
Rights Watch UK, a charity focused on national security measures, said in a tweet on Thursday it was "gravely concerned" by the proposals and the broader use of children as spies.
"Under domestic and international law, decisions which affect children must be taken in their best interests. Their welfare must be the primary consideration. It is difficult to imagine any circumstance where it would be in a child's best interest to be used as an informant," the group said.