Ms. Rudd promises a registration process much simpler than that now required to apply for permanent residency in the UK, and has brought on 1200 additional staff to speed up processing.

But processing 3M people by March 2019 is very likely not going to be doable.

Persons with criminal records, or using false identities, may be rejected.

EU citizens are suspicious of the ability of the Home Office not to make mistakes in its registration/rejection decisions.

Also, Home Office Permanent Secretary Philip Rutman said that in the event of a "no-deal" Brexit, as a last resort, troops could be assigned to patrol the UK borders.

The Brexiting process is going to have to be extended by 2 years or more to make this work.