The Home Secretary has pressed European leaders for details of arrangements for British expats seeking to remain in the EU after Brexit.
Sajid Javid said it is "currently unclear" what systems member states are creating to ensure the rights of UK citizens in their countries are protected after the end of the implementation period in December 2020.
Mr Javid raised his concerns as he provided an update on the progress of the settlement scheme for EU nationals living in Britain.
In a letter to Guy Verhofstadt, the European Parliament's Brexit Coordinator, he wrote: "As you would expect, the UK Government also cares very deeply about the reciprocal arrangements for UK nationals living in the EU27 Member States.
"We are concerned that as yet we have seen little information about the practical arrangements for securing their rights under the Withdrawal Agreement.
"Is the European Parliament able to provide any reassurance about progress on equivalent arrangements for UK nationals?
"We are discussing this issue with the Commission and I would encourage you to press for similar assurances to protect UK nationals."
Analysis published last year found that around 900,000 UK citizens were living in other EU countries.
UK officials are preparing from scratch an application scheme that will be open to more than three million EU citizens resident in the UK later this year.
Those who have been in the country 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.
People who have arrived by December 31 2020, but do not have five years residence, will be able to apply to stay until they have reached the threshold, at which point they can seek settled status.
In his letter, Mr Javid said: "Our default position will be to say 'yes' to applications."
An online application system will be accessible from any computer, laptop, smartphone or tablet with internet access.
As part of the process, applicants will be asked to confirm their identity.
The Home Office is developing an app that will mean people can confirm their ID and document authenticity without sending in documents or attending an appointment.
Mr Javid said the app can be downloaded for free onto any smartphone, but currently the ID verification element only works on Android devices.
The Home Office and Apple "continue to engage to see if we can achieve the same outcome for those with Apple devices", he added.
Applicants will also be able to complete the ID stage through the post.
Mr Javid said it is anticipated that applications between family members and dependants will be linked up to ensure "consistency of approach".
But he said the Home Office is proposing that individual family members should apply on separate forms, "reflecting the many different circumstances of modern families".
Applicants will be asked to disclose any criminal record, including overseas convictions.
"Where the applicant's conduct meets the conditions set out in the Withdrawal Agreement, their application for leave under the settlement scheme may be refused and deportation action considered," Mr Javid said.