The Government is facing calls to be investigated over claims ministers supporting the Remain campaign have used WhatsApp to discuss strategy away from official channels.
A private group on the encrypted messaging service has reportedly been used to exchange texts by people guiding the campaign to keep Britain in the EU, prompting allegations that details are being kept secret from the public.
According to The Sun, one senior Tory warned attempts to avoid transparency laws could lead to "scandal and embarrassment" for the Remain group, and Labour MPs are said to have called for the Information Commissioner to probe the claims.
WhatsApp is used by more than a billion people around the world to communicate by text and share media files, although it has come under scrutiny for its use of encryption that scrambles messages when they are sent between devices.
Eurosceptic Tory grandee David Davis told The Sun: "Attempts to avoid the Freedom of Information Act in what is supposed to be a legitimate activity will lead to scandal and embarrassment.
"This will only reinforce the public's view that the Remain campaign is prepared to cheat to win this referendum."
He warned that the claims echoed a similar scandal in the US where presidential hopeful Hillary Clinton became embroiled in a battle over the use of her private email.
The allegations against government figures supporting the Remain camp sparked concerns that the exchanges will not appear on official records that can be released to the public under the Freedom of Information Act.
What has the EU ever done for us?
What has the EU ever done for us?
1/7 1. It gives you freedom to live, work and retire anywhere in Europe
As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states.
2/7 2. It sustains millions of jobs
A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU.
3/7 3. Your holiday is much easier - and safer
Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed.
4/7 4. It means you're less likely to get ripped off
Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe.
5/7 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime
Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime.
6/7 6. Our businesses depend on it
According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business.
7/7 7. We have greater influence
Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.
Matthew Elliott, chief executive of the official Brexit group Vote Leave, told The Sun: "This is potentially criminal and should be investigated by Sue Gray, the Head of Ethics at the Cabinet Office and the Information Commissioner."
Emails sent from official Government accounts are archived and can be released to the public after varying periods have elapsed, depending on the sensitivity of the information.
Louise Haigh, Labour's shadow minister for digital industries, said using messaging services like WhatsApp could have implications for the 20-year rule of release of Cabinet papers.
She said she had "written to the Information Commissioner to ask that this is urgently investigated and either banned completely or brought under the FoI legislation."