A report suggests the Met is using a special technology to intercept people's mobile calls and even turn devices off.
Britain's largest police force is operating surveillance technology that can pose as a mobile phone network, according to a report.
This discreet technology transmits a signal allowing authorities to shut off phones, intercept communications and acquire data about thousands of users in specific areas, reports the Guardian.
The Metropolitan Police has reportedly gained access to the surveillance system from Leeds-based company Datong, which has customers including the US Secret Service, the UK's Ministry of Defence and Governments in the Middle East.
How can a device which invades any number of people's privacy be proportionate?
Datong has won over $1.6 million in contracts with US government agencies including the Secret Service, Special Operations Command and the Bureau of Immigration and Customs Enforcement.
Known as "Listed X," the technology can emit a signal over an area of up to 10 square km, which can acquire users' IMSI and IMEI numbers to then track their real-time movements, the paper suggested.
A transceiver the size of a suitcase is placed in a static location and operated wirelessly by officers. As an example of its use, this transceiver can be used to cut off a phone used as a trigger for an explosive device.
Under the Regulation of Investigatory Powers Act (Ripa), phone calls and SMS messages can be intercepted in the interests of national security.
Yet there is great concern among lawyers and privacy groups of innocent people being harmed by this information-gathering tool.
Jonathan Lennon, a specialist in cases involving covert intelligence and Ripa, said the Met's use of the Datong surveillance system raised legislative questions about privacy issues.
"How can a device which invades any number of people's privacy be proportionate?" Lennon said.
A spokesman for the Metropolitan police service said they use surveillance technology as a part of efforts to ensure the safety of Londoners and detect criminality, as it is a highly effective investigative tool.
Datong's products are sold in 40 countries around the world, including Eastern Europe, South America, the Middle East and Asia Pacific.