Tens of thousands of EU nationals living in Britain could become undocumented migrants after Brexit in a crisis bigger than the Windrush scandal, campaigners have warned.

It is claimed the Home Office’s system of requiring all EU citizens to apply for settled status ahead of Britain’s departure from the bloc could leave swathes of vulnerable people unable to prove their right to be in the country in years to come.

The EU Settlement Scheme, which opens today (21 January), is designed by the government to grant post-Brexit residency to EU citizens and their family members, who will have until 30 June 2021 to apply.

It will require applicants to prove their identity by using a mobile app which is part of an integrated online application process, at a cost of £65 for adults and £32.50 for children under 16.

But campaigners said that while the scheme may work for those who are aware of it, efforts to inform harder-to-reach people such as the elderly, those who don’t speak English and those who are being exploited, were “completely inadequate”.

And further criticism was levelled at the government over an app designed to process applications that can only be used on Android smartphones and tablets – cutting out Apple’s popular iOS devices. Other registration methods are available.

Chai Patel, legal policy director at the Joint Council for the Welfare of Immigrants, said: “They say they are doing advertising, that they are doing outreach and free events for people – but it’s completely inadequate for the situation at hand.

“If politicians meant it when they said EU nationals could stay, they would have made settled status a legal guarantee and said everyone who is currently a resident in the UK as an EU national has a permanent right to remain.”

Mr Patel accused ministers of using the scheme as a way to appear to “take back control” and remove a subset of people who they want to deport, but to the detriment of vulnerable individuals who will not be aware of it, or not have the means to apply.

“Ministers want it to be a political taking back control. They do not want to be seen to be granting settled status to people who are ‘undesirable’,” he said.

“But it means that we’ve got as a matter of absolute certainty at least tens of thousands of people who will end up being here illegally afterwards, just because they didn’t apply for the status – and that is going to be a problem.”

Britain before Brexit: West Midlands Show all 14 1 /14 Britain before Brexit: West Midlands Britain before Brexit: West Midlands Stoke A toy tiger, a long way from its natural habitat, no longer loved, no longer cuddled, caught in wires, lost within a maze of alleyways and red-brick terraces Richard Morgan/The Independent Britain before Brexit: West Midlands Coventry The Multi-Faith Peace Walk visits a gurdwara. The white man in the front row wears a cross, displaying allegiance to Christianity. His hat is a public sign of loyalty to Coventry City FC. There is no disrespect. On the contrary, this is an event that encourages multi-faith interaction, and his sincerity and concentration, I think, show that it’s working Richard Morgan/The Independent Britain before Brexit: West Midlands Birmingham Two outlines of man in British society, 100 years apart. Both carry a heavy load, both with a purpose, going somewhere, under orders, no doubt. But they’re split by the years between them, looking at life from different ends of the century, with different outlooks, facing different futures and fates Richard Morgan/The Independent Britain before Brexit: West Midlands Hanley There are small shops like this all over modern Britain. They are adorned with flags of the countries whose products you may find inside. They have names like “International”, “European”, “Baltic”, “Balkan”, “Eastern”, etc. “Unity Store” is exceptional, however, because it makes a political statement about the desirable relationship between the nationalities and their relationship to Britain Richard Morgan/The Independent Britain before Brexit: West Midlands Wolverhampton The blind leading the blind, or friends leaving the pub together, walking and talking together, going to the bus stop and heading home, not in need of a sighted leader Richard Morgan/The Independent Britain before Brexit: West Midlands Worcester A man sleeps in a pub, preserving body heat, his crossword incomplete and Guinness unfinished. There’s absence beside; a light emptiness that exaggerates the weight of his slumber Richard Morgan/The Independent Britain before Brexit: West Midlands Coventry Warriors in the heat of a Pokémon battle, war-weary, yet glued to the action, fighting on. Reality is up for grabs in this scene, fragmented across three planes: the street, the battle, and the airbrushed projections of femininity Richard Morgan/The Independent Britain before Brexit: West Midlands Worcester A crowd looks on as a young man contemplates suicide, standing atop a building in the town centre. A fireman eventually persuades him not to jump, and he is carried down to earth by the mechanical arm of a fire engine Richard Morgan/The Independent Britain before Brexit: West Midlands Birmingham Jehovah’s Witnesses try to convert people outside Moor Street station. They compete for attention with a sign promising financial gain. There’s a choice here, between the world of faith and the heady skylines of Birmingham and London, where what’s in your pocket matters most Richard Morgan/The Independent Britain before Brexit: West Midlands Worcester A church on the high street undergoes construction. Scaffolding is hidden by boards displaying messages about God and Christianity. Coupled with a secular, national symbol, the text takes on another meaning, a commentary on the nation itself as a broad church, as shelter, as a place of worship Richard Morgan/The Independent Britain before Brexit: West Midlands Coventry Skeletons behind curtains in a spiritualist church. They are there for Halloween, but it looks like a joke about the role of the dead in the church, who are kept close, communicating with terrestrial beings through the energy that binds all things Richard Morgan/The Independent Britain before Brexit: West Midlands Warwick Red poppies made from plastic bottle bottoms. Recycling in the service of memorialisation. The act of remembrance blurs with the ethics of 21st century environmentalism Richard Morgan/The Independent Britain before Brexit: West Midlands Coventry A tent pitched in the middle of the city, next to the pavement, cramped in a corner. The inhabitants’ towel hangs out to dry, below a cruel, unintentional, chance commentary on the transitory, temporary, and thoroughly unhomely settlement. A boy moves in the light, playing or hiding or turning away Richard Morgan/The Independent Britain before Brexit: West Midlands Warwick An early morning view of Warwick castle, still black, just reappearing, yet the flag shines as if it had done so the whole night through, as if it had always been this way; a beacon of light in a world of darkness Richard Morgan/The Independent

Critics have also warned that thousands of people could be left without legal status if applications are not processed quickly and efficiently.

Jill Rutter, think tank British Future’s strategy director, said: “The Home Office must invest in getting the EU settlement scheme right from the start.

“Failure to do so could cause massive problems in years to come, on a far bigger scale than the Windrush scandal.”

Charities said that the fee to register, although “not huge”, would also deter some people from applying, and criticised the Home Office for refusing to introduce a fee waiver for those who may not be able to afford it.

In November, the government came under fire after immigration minister Caroline Nokes admitted there were no plans to waive fees for European trafficking survivors in the UK to secure their status – which modern slavery campaigners said risked pushing them back into exploitation.

A petition calling on the Home Secretary Sajid Javid to axe the fee has already attracted 6,000 signatures. Chief executive of charity More United, Bess Mayhew, said: "Asking EU citizens who have lived here for years to pay to stay in their homes isn’t right.

"These people are our friends, family and colleagues who deserve to be treated better. We hope there is still time for the Home Secretary to reconsider."

Liberal Democrat MP Sir Ed Davey said: “No one seriously believes that the Home Office will be able to grant settled status to everyone who’s eligible within two years. Thousands will be left effectively undocumented and subject to Theresa May’s hostile environment.”

Those who have lived continuously in the UK for five years can apply for settled status under the scheme, meaning they are free to go on living and working in the UK indefinitely.

People who do not have five years’ residence can seek to stay until they have, at which point they can apply for settled status.

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Applicants are asked to prove their identity, declare any criminal convictions and upload a facial photograph. Officials then check employment and benefits data to confirm proof of residence, while all applications are run through UK criminality and security databases.

It is estimated that the total number of applications could run to more than 3.5 million.

A Home Office spokesperson said: “The EU Settlement Scheme will ensure all EU citizens living in the UK will have a status granted by the Home Office which they can use to demonstrate their right to work, housing and benefits.

“It will be simple and straightforward for EU citizens to get the status they need. They will only need to complete three key steps – prove their identity, show that that they live in the UK and declare any criminal convictions.

“We have completed two successful private testing phases with tens of thousands of applications and customer feedback was overwhelmingly positive.