The Home Office is being warned it risks creating a new “Windrush scandal” as the post-Brexit scheme to register an estimated 3.5 million European Union citizens living in the UK begins.

EU nationals and their family members who wish to remain in the country beyond June 2021 must apply to the settlement scheme, which enters its first public testing phase from today.

READ MORE: Lessons to be learned from the Windrush affair

The website and app opens to EU nationals living in the UK with passports and their non-EU family members with biometric residence cards, ahead of a full launch by April.

However, critics are warning thousands could be left without legal status if applications are not processed quickly and efficiently.

Jill Rutter, strategy director of think-tank British Future, 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.”

According to the Office of National Statistics, there are about 228,000 EU citizens living in Scotland, including Irish, who will be exempt from the rule.

Immigration Minister Caroline Nokes said extensive testing “shows clearly we are well on track to deliver a system that will make it easy and straightforward for EU citizens to obtain status”.

READ MORE: Herald View: Home Office has to get this right and avoid another cruel Windrush-style fiasco

But pressure group the3million surveyed its EU members to find their biggest concern was losing their rights in the future, with founder Maike Bohn warning that trust in ministers is low.

“The Windrush people trusted the Home Office and many of them got deported because they were citizens but couldn’t prove it,” she said.

Officials expect they can process about 6,000 applications a day, with about 1,500 caseworkers on the scheme and a further 400 in a resolution centre to deal with issues.

A trial of nearly 30,000 applicants was restricted to people in specific professions, with only a small number of vulnerable people participating.

More than two-thirds were approved in three working days and 81 per cent within a week.

While improvements to the process have been made, nearly one-quarter of people told the Government they found it difficult during previous testing.

The price of applying for settled status is £65 for adults and £32.50 for children, although people already granted permanent residence face no extra charge.

The Scottish Government is already in a stand-off with Westminster overpaying the £65 settled status fee for those working in the public sector in Scotland.

Many will choose to use an app created by the Government, but it can only be used on Android devices, which excludes iPhones and iPads, although there other ways to apply.

British Future’s report, entitled Getting It Right From The Start: Securing The Future Of EU Citizens In The UK, has identified potential barriers. It says people may not hear about the scheme, might not realise it applies to them, struggle to provide proof of residency or find the system hard to navigate.

There is also a possibility the system will have technical difficulties matching

names and official records, according to the report.

Its co-author Ms Rutter said: “The stakes are high. Get it right and the UK sends a strong message that EU citizens are welcome and the Government is in control. Get it wrong and the consequences are dire.”

British Future suggests a “cost-price” British Citizenship offer, at a reduced rate of £300, to EU citizens with five years’ continuous residency, who meet the other citizenship requirements.

Liberal Democrat MP Ed Davey said: “No one seriously believes 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, 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 seek settled status.

Applicants are asked to prove their identity, declare any criminal convictions and upload a facial photograph.

Officials check employment and benefits data to confirm proof of residence, while all applications are run through UK criminality and security databases.

It is anticipated that the total number of applications could run to more than

3.5 million.

A Home Office spokesman added: “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 they live in the UK, and declare any criminal convictions.”