Ministers could be banned from using platforms such as WhatsApp and Signal for formal communications, if a legal challenge is successful.
Campaigning law group Foxglove, on behalf of transparency campaigners The Citizens, has accused the government of having no legal policies on “government by text”.
Members of both the cabinet and shadow cabinet use WhatsApp and Signal.
The Cabinet Office has been contacted for comment.
But previously, a spokesman for the Cabinet Office told the BBC that “appropriate arrangements” are already in place to adhere to guidance, and “this is kept under periodic review”.
Some instant messaging services allow you to permanently delete messages, which critics say politicians and staff could use to avoid accountability.
Campaigners have threatened to sue the government if it does not give a clear explanation of its policies on such messaging platforms, and how they adhere to the law, within 14 days.
Last month, they sent a legal letter with a similar request for transparency over these kinds of communications, but did not receive an individual response.
This new letter is a formal pre-action notice and final opportunity to respond, before launching the court case.
According to official guidance, which has existed since 2013, a record only needs to be retained “if it is needed for substantive discussions or decisions in the course of conducting business”.
For example, this law would apply to messages exchanged between special advisers and a minister regarding government policy.
UK law requires that such messages be archived to record, and it is up to the originator or recipient of such messages to “take the steps” to ensure this is done.
Messages on such platforms can be set to self-destruct, or be permanently deleted – meaning they could not be stored as a public record, or revealed in a later Freedom of Information (FOI) request.
“A government of WhatsApp and Signal is undemocratic and unlawful,” said Kerry Shaw, of The Citizens.
“It’s enabling the rampant cronyism and sleaze that’s infecting this government.
“We’re seeing a wholesale theft of evidence that belongs to the people – and to history.”
Last month, the group sent legal letters to the government, asking for clarification on these issues.
The Department for Digital, Culture, Media and Sport responded with a blanket FOI acknowledgment letter, which Foxglove said was a “play for time”.
The challenge adds further scrutiny on the cabinet’s use of mobile phones for official government communications.
No 10 has launched an inquiry after leaked text messages – first reported by the BBC – showed Mr Johnson saying he could “fix” tax issues relating to Dyson staff who came to the UK to work on the pandemic.
That came not long after the Treasury released two of its text messages sent by Chancellor Rishi Sunak, amid an ongoing lobbying row.
The Institute for Government think tank has also campaigned for all messages sent and received on official phones to be logged.
“There needs to be a public record that something’s happened,” associate director Tim Durrant told the BBC. “Texting is essentially a conversation, like a meeting.”