Tens of thousands of EU migrants could lose their right to be in the UK after Brexit - and the authorities will not know who they are, a new report warns.
EU citizens must register using an online system to secure "settled status" when the UK leaves next March.
The government has said it expects about 3.5m applications.
But the Migration Observatory said ministers had no precise figures for how many EU citizens were living in the UK and how many plan to stay.
The think tank, which is based at Oxford University, has previously warned 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."
Last month, Home Secretary Sajid Javid launched what he a called a "simple" smartphone-based system, to enable EU citizens to carry on living in the UK legally when Britain leaves the EU.
Applicants will be asked to provide their biographical information, declare whether they have any criminal records, and upload a facial photograph - and pay £65 for adults and £35 for children.
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 31 December 2020, but do not have five years' residence, can seek to stay until they have, at which point they can seek settled status.
Migration Observatory director Madeleine Sumption, 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."
The think tank calls on the Home Office to establish a clear plan for measuring whether EU nationals living in the country have received "settled status".
A Home Office spokesman said securing citizens' rights was a priority on which it had delivered.
"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," they said.
"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 UK migration figures on Monday.