Charities and campaign groups have expressed growing alarm at the number of European migrants who may be ignorant of the need to apply to remain in Britain after a June deadline imposed as part of the country’s post-Brexit immigration system.

Although the Home Office in December said it had received 4.9m applications for the UK’s settled status scheme — 1.1m more than the number it predicted back in 2018 — campaigners said many more remain unaware, risking thousands of eligible people being cut off from rights to live, work and claim benefits in Britain.

“From our experience, we’re very worried and concerned that we won’t reach everybody,” said Mags Brady, chief executive of PBIC, a charity in Bedford, north of London, that helps eastern European migrants with issues such as learning English and employment rights as well as applying for the EU settled status scheme.

Ms Brady added that each week, PBIC had been finding an average of four people who contacted it in connection with other problems were ignorant of a looming risk to their right to remain in the UK.

With the UK’s new post-Brexit immigration system coming into force from January 1, the EU Settlement Scheme is one of the many adjustments Britain and the EU are having to make now that freedom of movement has ended along with the UK’s membership of the single market and customs union.

The settlement scheme offers citizens of the 27 EU member states as well as Switzerland, Norway, Iceland and Liechtenstein the right to keep living and working in the UK as they did when the country was part of the EU.

Applicants who can prove they have lived for five years continuously in the UK are awarded settled status, which grants full access to rights such as unemployment benefits. Other applicants receive “pre-settled status”, which confers fewer rights but should convert into settled status after five years of continuous residency.

The Home Office, which is running the scheme, insists the programme has been “hugely successful”, but groups working on the issue worry that even those who are aware of the need to apply are struggling to master the technology or muster the evidence required. Applicants have until June 30 to sign up.

Assessing the true number of European migrants who remain and are at risk of being left in legal limbo after that date is made all the more difficult because of a lack of a central record. Before Brexit, people born in the EU were free to live and work in the UK without needing to meet new requirements to earn at least £25,600 or to have certain skills specified under Britain’s immigration system.

Kevin Foster, minister for future borders and immigration, last month hailed the number of applications the Home Office had received to the settlement scheme so far, calling it “terrific news”.

But while Chai Patel, legal policy director of the Joint Council for the Welfare of Immigrants, accepted the scheme had attracted more applications than expected, he warned: “All that this has really proved is that the Home Office were way off base in the number of people that they need to reach — and that’s not reassuring.”

At the heart of the scheme’s success — and its potential problems — is the innovative technology that the Home Office has developed to handle applications.

The scheme depends on a flexible, online system that offers applicants the option of using smartphones to scan in their identity documents. The system scours government records for tax returns and other evidence of a person’s claim of residence in the UK. The system has enabled the Home Office to make decisions on 4.3m of the 4.9m applications since the scheme opened in March 2019.

However, the technology has also been a barrier for some applicants.

Joanna Karwecka, an immigration adviser working for the East European Resource Centre in Hammersmith, west London, said applicants she was helping often lacked any kind of internet access. Like other support groups, EERC was currently unable to offer face-to-face consultations because of coronavirus restrictions.

EERC and PBIC are among 72 organisations throughout the UK that the Home Office has funded to support vulnerable people through the application process.

“The people we’re in contact with don’t have proper internet access or a mobile [phone] with cameras,” Ms Karwecka said. “Those people are waiting for us to open the office so that they can come and see us because they’re not able to pass through this very basic first step.”

There are particular concerns that the most recent statistics showed fewer than 600,000 applications from under-18s, against previous estimates that there were about 900,000 eligible children living in the UK.

Marianne Lagrue, policy manager at Coram Children’s Legal Centre, a London-based charity that has also received Home Office funding, said evidence of the right to remain in the UK could be especially hard to find for children in local authority or foster care. Language barriers and other problems also made some families especially hard to reach.

“It’s exactly the same problem as with any other statutory engagement,” Ms Lagrue said. “The families who are engaged and on it are easy to find and the families that aren’t are not.”

Case workers also pointed to numerous other groups — including older people who may have lived in the UK for decades and agricultural workers living on farms — as hard to reach with the Home Office’s message.

The Home Office has insisted that it will take a “pragmatic and flexible approach” to assessing any circumstances that cause someone to miss the June 30 deadline — and that it will regard a child as having a reasonable excuse for having missed it if a parent failed to apply on his or her behalf.

Yet the lack of firm guarantees continues to worry many organisations helping migrants. Bill Tidnam, chief executive of Thames Reach, a homelessness charity that has also received Home Office funding, pointed out that its clients by the nature of their lives lacked ready access to the information required for applications.