According to data reviewed by the Observer, almost 4 million non-EU immigrants residing in Britain will have to convert to digital "eVisas" by the end of the year or risk losing their ability to demonstrate their legal status.

All foreign nationals who are permitted to reside in the UK for a minimum of six months are issued Biometric Residence Permits (BRPs), which serve as documentation of their eligibility to pursue education, utilise public services, and make benefit claims. However, as part of the Home Office's digitization initiative, they are being replaced.

Data from the Home Office released under the Freedom of Information Act revealed that 4,066,145 people have BRPs that expire on 31 December, despite having legal leave to remain in the UK beyond that date. By the end of this year, they will need to have replaced these physical permits with digital eVisas.

The Home Office has already struggled in early attempts to contact those affected, as in some cases the email address they have on file is for a migrant’s lawyer, and charities fear migrants could fall foul of the “cliff edge” deadline.

To access their eVisa, people will need to open a UK Visas and Immigration (UKVI) digital account. People can still apply for this account after 31 December, but if they are not aware of the new rules they may only discover this when they are unable to prove their right to return from holiday or claim benefits.

Campaigners are warning that the next government must address the 31 December deadline or risk a disaster.

The figures were secured from the Home Office by the Helen Bamber Foundation, a human rights charity. Zoe Dexter, its housing and welfare manager, said: “It is shocking that the Home Office is rushing through their digitisation plan which affects millions of people in the UK in a ridiculously quick and chaotic manner.

“A huge number of people are at risk of not being able to prove their right to work, rent or travel. Many people will unnecessarily lose out on vital benefits because the Department for Work and Pensions also appear to have no plan for implementing this digitisation of IDs.

“This will adversely affect people who are particularly vulnerable, including refugees and survivors of trafficking and torture.

“Whoever is in power after the election will have to sort out this huge issue and will need to ensure that all government departments coordinate better so that millions are not at risk of significant injustices.”

The Home Office told the Observer that the 4.06 million figure includes “some” BRP holders who already have an eVisa. However, since the eVisa rollout only began in mid-April, this number is likely to be limited.

Josephine Whitaker-Yilmaz, of the migrant rights charity Praxis, said: “Millions of people will be left with no way to prove their rights if the next government ploughs ahead with the transition to a digital-only visa system by the end of this year.”

Whitaker-Yilmaz added: “The Windrush scandal showed us clearly what can happen when people have their ability to prove their rights taken away by Home Office blunder.

“Without a change of course, potentially millions of people will find themselves unable to rent a property, get a job or access essential services.”