A Green Party peer has urged a whistleblower to assist investigators probing claims undercover counter-extremism police used hackers in India to access the emails of journalists and environmental activists.
A tip-off from an anonymous letter sparked the investigation into the Metropolitan Police-run National Domestic Extremism and Disorder Intelligence Unit (NDEDIU), the Independent Police Complaints Commission (IPCC) 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.
The Met said it taken "all possible steps" to keep material which could be relevant to the inquiry.
Jenny Jones said she had been sent the letter and had it checked out by human rights lawyer Jules Carey, who found some of those named confirmed their passwords matched the ones listed in the document.
Baroness Jones of Moulsecoomb said she was not named in the list and did not believe she was among the alleged hacking victims.
She previously alleged that records relating to her were destroyed or deleted in June 2014 and her case is one of another two ongoing IPCC investigations into NDEDIU over alleged document shredding.
"I really welcome the IPCC taking this action," Lady Jones said.
"It's incredibly difficult to deal with that in any organisation let alone the police. It's such a crucial issue because if people don't trust the police then people won't come to them."
She said it was "utterly appalling" that activists may have been targeted, adding: "It's inappropriate they should be monitored in that way."
Of her own position, she added: "I could not care less about being hacked or monitored - I'm a politician. I expect scrutiny, I've got nothing to hide."
Lady Jones said she almost discarded the anonymous letter at first before realising its significance and reporting it to the IPCC.
"Normally I would put it straight in the bin but there was enough in it," she said.
"This was utterly coherent and clearly checked out.
"The letter held up - it said this list was just one day and so there must have been hundreds, I'm guessing.
"It takes a lot of bravery to come forward."
Greenpeace campaigners are among those thought to have been named in the letter, while the Guardian reported two of its journalists were said to have allegedly had their emails monitored.
John Sauven, Greenpeace UK's executive director, said: "Given the seriousness of the allegations, we welcome the announcement of an investigation by the Independent Police Complaints Commission.
"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?
"Greenpeace is an entirely peaceful organisation, our staff even give lectures at the police training college in Hendon on how best to manage protests so people's rights are protected. The hacking of Greenpeace staff emails would be further evidence that the Met police undercover unit was allowed to spin out of control."
A Guardian News & Media spokesman said: "We welcome the IPCC 's decision to investigate allegations that the Metropolitan Police accessed the email accounts of journalists and campaigners outside of the established legal framework.
"However, we are disappointed that neither organisation has responded to our requests for information on journalists affected. The security of the communications of reporters is central to a free press, which is absolutely vital to hold power to account.
"We await the outcome of the IPCC's investigation with interest."
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."
A Met Police spokesman 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."