A claim that undercover counter-extremism officers used hackers in India to access the emails of journalists and environmental activists is being probed by the police watchdog.
A tip-off from an anonymous letter sparked the investigation into the Metropolitan Police's National Domestic Extremism and Disorder Intelligence Unit (NDEDIU), the Independent Police Complaints Commission said.
The letter alleged covert officers from the unit contacted Indian officers for help to enlist the services of hackers, who illegally accessed the email accounts of members of political and environmental pressure groups and journalists.
The watchdog has appealed for the author of the letter, believed to be a serving or retired police officer, or anyone with information on the allegations, to contact investigators.
IPCC deputy chairman Sarah Green said: "These are clearly serious allegations and the IPCC is conducting a comprehensive investigation into the matters raised.
"This will be a complex investigation given the potential involvement of foreign participants.
"We would like to hear from the officer who brought these allegations to light or any other officers or police staff who may be able to provide information of use to the IPCC investigation."
The police watchdog already has two related and ongoing investigations into allegations that paperwork relating to undercover policing kept by the NDEDIU was shredded in May 2014.
The Met Police said: "The IPCC made the Metropolitan Police Service (MPS) aware of anonymous allegations concerning the access of personal data and requested the matter referred to them by the MPS. This has been done.
"The MPS is aware that the IPCC is carrying out an independent investigation.
"As this investigation is now in the public domain the MPS can confirm that all possible steps are being taken to ensure all relevant material and associated computer systems are preserved to assist the IPCC investigation."