Undercover counter-extremism officers used hackers in India to access the emails of journalists and environmental activists, it has been claimed.
The police watchdog started an inquiry into claims against the Metropolitan Police after an anonymous tip-off.
It appealed for the whistleblower – believed to be a serving or retired police officer -to get in touch.
The Met said it was aware of the Independent Police Complaints Commission’s (IPCC) investigation.
The IPCC said it had received an anonymous letter, which alleged covert officers from the Met’s National Domestic Extremism and Disorder Intelligence Unit (NDEDIU), contacted Indian police officers for help to enlist hackers.
The letter alleges the hackers accessed the email accounts of hundreds of people, including members of political and environmental pressure groups and journalists.
Greenpeace was one of the organisations believed to have been named in the letter.
John Sauven, Greenpeace UK’s executive director, said the charity welcomed the announcement.
“If the allegations are true, the public and our campaigners deserve to know who ordered the hacking of our staff, why an overseas company was used to break into their emails, who else was targeted and what was done with the information,” he said.
The IPCC is already carrying out two related and ongoing investigations into allegations that paperwork relating to undercover policing kept by the NDEDIU was shredded in May 2014.
One of those investigations follows allegations made by Green Party peer Baroness Jenny Jones that records relating to her were destroyed or deleted in June 2014.
‘Relevant material preserved’
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 Met Police said: “The IPCC made the Metropolitan Police Service 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.”