At least 26 police forces in England and Wales have begun using new technology to extract data from phones.
And Privacy International said there had been no public debate about the rapid rollout of this practice.
But one former chief constable said obtaining a warrant in each instance would be “just not practical”.
Privacy International obtained the figures through Freedom of Information requests to 47 forces, of which 42 responded.
The extraction devices used generally take everything of one type off a phone – so if a witness’s mobile contains a photo important to an investigation, the device will download all photos.
The National Police Chiefs Council said the decision to download phone data was a judgement that could be made on a case-by-case basis “defined by the investigative requirements of the case”.
But Privacy International said it feared there was no national oversight, and no clear guidance on when to delete the data.
Of the 47 UK police forces it contacted, only eight said they had their own local guidance about using this technology.
And Derbyshire and Wiltshire Police’s guidance allows the downloading of a phone’s contents without the suspect’s knowledge.
Privacy International said requiring a warrant, like those needed to search someone’s home, would mean any police request to access phone data would be subject to independent judicial oversight.