More than two million EU nationals and their family members have asked to remain in the UK after Brexit following a spike in applications which saw half a million people apply for settled status last month alone.

Government figures show that in September, 520,600 European citizens and their relatives had applied for the right to continue their legal residence in the UK after the country leaves the bloc – the highest monthly figure since the EU Settlement Scheme opened in April, up from 299,000 the month before.

Of all applications received so far, 18 per cent – or 335,700 – have not yet been concluded, prompting concern among campaigners that cases are becoming more complex and that people may be experiencing difficulty proving their residence in the UK.

Concern has also been raised at the fact that almost four in 10 applicants (38 per cent) were granted only pre-settled status, requiring them to reapply for permanent status once they have been in Britain for five years.

Campaigners said this meant people may have been wrongly denied permanent immigration status, particularly after a number of high-profile incidents of this emerged in the media recently.

It comes after a leaked draft resolution seen by The Independent last month revealed MEPs were preparing to express “grave concern” about the EU settlement scheme and demand that it be subject to a full review.

Separately, The Independent revealed that around 5,000 vulnerable EU national children in Britain face losing their rights after Brexit because the Home Office has been providing “woefully inadequate” support for them to apply.

Maike Bohn, co-founder of campaign group the3million, said the spike in applications in September was due to EU citizens being “terrified” of the consequences of a no-deal Brexit, and described the increase in the backlog in undecided applications as “worrying”.

“It indicates that cases are becoming more complex and people might have more and more difficulties evidencing their residence in the UK,” she said.

“Having said that, the3million is worried about the hundreds of thousands of EU citizens who won’t be able to apply by December 2020, and the UK government has yet to declare how these people who are living lawfully in the UK will be able to continue to live their lives.

“Elderly residents in care homes, people with disabilities, children and those who won’t know they have to apply are at risk of tipping into the hostile environment after the hard deadline unless the government says explicitly how it will handle those cases.”

Ms Bohn called on ministers to make the system declaratory so that people who qualify automatically would be granted status without having to evidence it, saying this would prevent a “Windrush on steroids” scenario.

Christine Jardine, Liberal Democrat home affairs spokesperson, said: “Boris Johnson has broken the promise he made to the 3.6 million EU citizens in the UK. He said their rights would be automatically guaranteed in law, but today’s figures show that the process is anything but automatic.

“If the Conservatives carry on with their Brexit plans, thousands of EU citizens will miss out on Settled Status and be left vulnerable to the Tories’ hostile environment. Brexit will create an even bigger Windrush-style scandal.”

She said the fact that hundreds of thousands of people were being given “second-class” pre-settled status meant they would face a “new cliff-edge” in five years, describing it as “another disaster waiting to happen”.

Jacob Rees-Mogg claims 'one million' EU nationals have been granted settled status in the UK

Home secretary Priti Patel celebrated the figures, saying: “EU citizens have made a huge contribution to this country and will play a key role in cementing Britain’s status as an outward-looking, global leader after Brexit.”

Security minister and deputy for EU exit and no deal preparation Brandon Lewis meanwhile said: “There is plenty of support and information on offer to help people apply and get the status they need.