The UK Home Office’s new mobile phone app, which is intended to replace an 85-page printed application for registering European Union citizens in the UK, has been put on hold. British Prime Minister Theresa May has locked horns with Brussels over people arriving during the transition period.

The new electronic system was ready to be road-tested in January but the Home Office has had to go back to the drawing board after the prime minister warned there would be no automatic right to remain for EU citizens coming to the UK after 29 March 2019, the Guardian revealed.

“After Theresa May’s announcement about EU citizens, the certainty about the system has been taken away,” one source told the Guardian.

Campaign group the3million, which has been lobbying for status quo rights for EU citizens already settled in the UK, fears problems. The elderly, for instance, may not have computers. What about the infirm and “the scared”?

“What will happen if the app rejects you, what will happen if you don’t have a computer, if you are disabled? What will happen if the Home Office makes a mistake? All of this will be an issue. It will be only work for the digital generation,” said Nicolas Hatton, co-founder of the3million. “We will still need a face to face solution alongside this.”

He predicted that some people would refuse to register out of fear or principle.

Speaking to the Guardian, a spokesman for the Home Office explained that it would be providing assistance for those who could not make an online application.

Immigration lawyer Ian Robinson, a partner at law firm Fragomen, warned the system might not be able to cope if millions apply to register on day one.

“The trick will be to manage demand because it won’t help anyone if they get 2m applications on day one, only for the system to fall over.”