Three of the UK’s four big mobile phone networks have made customers’ call records available at the click of a mouse to police forces through automated systems, a Guardian investigation has revealed.

EE, Vodafone and Three operate automated systems that hand over customer data “like a cash machine”, as one phone company employee described it.

Eric King, deputy director of Privacy International, a transparency watchdog, said: “If companies are providing communications data to law enforcement on automatic pilot, it’s as good as giving police direct access [to individual phone bills].”

O2, by contrast, is the only major phone network requiring staff to review all police information requests, the company said.

Mobile operators must by law store a year of call records of all of their customers, which police forces and other agencies can then access without a warrant using the controversial Regulation of Investigatory Powers Act (Ripa).

Ripa is the interception law giving authority to much of GCHQ’s mass surveillance. The law was again under the spotlight recently after it was used to identify sources of journalists from at least two national newspapers, the Sun and the Mail on Sunday.

Documents from software providers and conversations with mobile companies staff reveal how automatic this system has become, with the “vast majority” of records demanded by police delivered through automated systems, without the involvement of any phone company staff.

The Home Office argues communications data is “a critical tool” and its use of Ripa was “necessary and proportionate”.

Despite politicians’ assurances that the UK laws requiring phone companies to keep records would not create a state database of private communications, critics argue that the practice comes very close to doing so. King warned that “widespread, automatic access of this nature” meant the UK telecoms industry “essentially already provides law enforcement with the joined-up databases they claimed they didn’t have when pushing for the ‘snooper’s charter’.”

In the automated systems used by the phone companies, police officers seeking phone records must gain permission from another officer on the same force, who then enters the details into an online form. That mirrors the US Prism programme, revealed by Edward Snowden, which in effect created a backdoor into the products of US tech corporations. In the vast majority of cases, the information is then delivered without any further human role.

One document prepared by Charter Systems, which sells the type of software used by police forces to connect with mobile phone companies, explains the automated process saves “32 minutes” of human time per application.

“Charter Systems have worked in partnership with the Home Office and Detica [a firm providing data interception for security services and the police, now called BAE Systems Applied Intelligence] to develop a solution that links directly to all CSPs [communication service providers, a term covering phone companies],” it states. The document explains the system produces “an automated solution for gathering electronic data information. The new solution saves time and effort for the authority in requesting and receiving ever increasing amounts of data.”

The systems were so interconnected, a separate sales document produced by Charter reveals, that “[d]ata can be retrieved from multiple CSPs in one request”.

Privacy groups reacted angrily to the details of how little day-to-day scrutiny records requests receive, warning that the automation of the system removes even the limited oversight ability – the right to refer requests to oversight agencies – phone networks have over Ripa requests.

“We urgently need clarity on just how unquestioning the relationship between telecommunications companies and law enforcement has become,” said King. “It’s crucial that each individual warrant for communications data is independently reviewed by the companies who receive them and challenged where appropriate to ensure the privacy of their customers is not being inappropriately invaded.”

Privacy advocates are also concerned that the staff within phone companies who deal with Ripa and other requests are often in effect paid by the Home Office – a fact confirmed by several networks – and so may, in turn, be less willing to challenge use of surveillance powers.

Several mobile phone networks confirmed the bulk of their queries were handled without human intervention. “We do have an automated system,” said a spokesman for EE, the UK’s largest network, which also operates Orange and T-Mobile. “[T]he vast majority of Ripa requests are handled through the automated system.” The spokesman added the system was subject to oversight, with monthly reports being sent to the law enforcement agency requesting the data, and annual reports going to the interception commissioner and the Home Office.

A spokesman for Vodafone said the company processed requests in a similar way. “The overwhelming majority of the Ripa notices we receive are processed automatically in accordance with the strict framework set out by Ripa and underpinned by the code of practice,” he said. “Even with a manual process, we cannot look behind the demand to determine whether it is properly authorised.”

A spokesman for Three, which is also understood to use a largely automated system, said the company was simply complying with legal requirements. “We take both our legal obligations and customer privacy seriously,” he said. “Three works with the government and does no more or less than is required or allowed under the established legal framework.”

Unlike the other networks, O2 said it did manually review all of its Ripa requests. “We have a request management system with which the law enforcement agencies can make their requests to us,” said the O2 spokeswoman. “All O2 responses are validated by the disclosure team to ensure that each request is lawful and the data provided is commensurate with the request.

Mike Harris, director of the Don’t Spy On Us campaign, said the automated systems posed a serious threat to UK freedom of expression. “How do we know that the police through new Home Office systems aren’t making automated requests that reveal journalist’s sources or even the private contacts of politicians?” he said.

“Edward Snowden showed that both the NSA and GCHQ had backdoor access to our private information stored on servers. Now potentially the police have access too, when will Parliament stand up and protect our fundamental civil liberties?”

A spokesman for the Home Office declined to respond to specific queries about the use of automatic systems to retrieve call records, but defended police forces’ use of Ripa. “Communications data is an absolutely critical tool used by police and other agencies to investigate crime, preserve national security and protect the public,” he said in a statement.

“This data is stored by communications service providers themselves and can only be acquired by public authorities under the Regulation of Investigatory Powers Act 2000 on a case by case basis, and where it is necessary and proportionate to do so.

“The acquisition of communications data under RIPA is subject to stringent safeguards in existing legislation and is independently overseen by the Interception of Communications Commissioner.”