Millions of EU citizens now have until June 2021 to apply for settled status in the UK, as the post-Brexit registration system opens nationwide.
The government say it has made the £65 process as easy as possible, developing a smartphone app, hiring 1,500 new caseworkers and investing £175m in the scheme.
But it is feared it could sow the seeds of a new Windrush scandal with hundreds of thousands of people potentially unaware or unable to register their right to remain in the UK.
Think-tank British Future warned that of the 3.5 million Europeans currently living in the UK, one third could struggle to properly make an application.
It warns that vulnerable groups like the elderly, people with limited English or IT skills, or people isolated from the wider community may be particularly affected.