Tens of thousands of EU migrants could lose their right to be in the UK after Brexit without showing up in official data, according to a new report.

It warned that unless the Government has a clear plan for measuring whether EU nationals living in the country have received “settled status”, the precise number who fall through the gap may never be known.

The paper from the Migration Observatory at the University of Oxford said that based on currently available statistics, it will not be easy to calculate exactly how many miss out unless the numbers are very large.

“This is because we do not have precise figures on how many EU citizens are living in the UK and plan to stay,” the report said.

“It is possible that tens of thousands of EU citizens living in the UK could lose their legal status without this being clear from the data.”

Later this year the Government will launch an online application system for EU nationals seeking to remain in the UK following Brexit.

EU citizens and family members who have been in the UK for five years by the end of 2020 will be able to apply for settled status, meaning they are free to go on living and working in the UK indefinitely.

Those who have arrived by December 31 2020, but do not have five years’ residence, can seek to stay until they have, at which point they can seek settled status.

The Migration Observatory’s report said that in principle it should be possible to assess whether the Home Office is on track to document the whole population of EU citizens living in the UK by looking at how many grants of settled or pre-settled status have been made after the scheme opens.

But in practice this will not be entirely straightforward, the paper warned.

It said there is no “list” of EU nationals living in the UK and estimates of their numbers are not precise.

The Migration Observatory has previously raised concerns that thousands of EU citizens could inadvertently become illegal residents in the UK after Brexit despite meeting the required criteria to stay.

Its latest report said: “A whole host of factors, from lack of awareness to fear of rejection to simple disorganisation mean that some eligible EU citizens will not apply.”

Madeleine Sumption, director of the Migration Observatory, said: “It’s reasonable to expect that even with a perfectly designed application and a great communications campaign, some EU citizens will fall through the gaps and fail to secure settled status.

“Government and civil society will want to know who and how many so that they can address the problem.”

She said generating new data could “save a tremendous amount of effort later on”, adding that this requires advance planning “right now”.

A Home Office spokesman said: “Securing the rights of citizens has always been our priority and we have delivered on this commitment.

“The draft Withdrawal Agreement published in March guarantees the rights of EU citizens and their family members living in the UK, and of UK nationals living in the EU.

“But we recognise that we need to reach out to and support a wide range of people including vulnerable groups whose needs will vary, such as the elderly, those who cannot access or aren’t confident with technology and non-English speakers.”

The Office for National Statistics will publish the latest figures on migration to the UK on Monday.