The settled status deadline for EU citizens saw a five-fold increase in applications on the final day as 50,000 people applied before midnight on Wednesday.
The last-minute scramble was so great that the Home Office extended the time applications would be accepted to 9am on Thursday.
It was "similar to the peak" day just before the end of the transition period on Dec 31, when 58,000 applied. That compares with a daily average of 10,000 to 12,000 in the past month, according to Home Office briefings last week.
The surge came after Boris Johnson on Wednesday urged eligible EU citizens to "get on with" applying amid concerns that tens of thousands could miss the deadline.
Experts and community leaders estimate that between 100,000 and 500,000 EU citizens eligible for settled status may not have applied by Wednesday’s deadline. Some 5.6 million have so far applied for settled status, two million more than originally anticipated.
Failure to apply could ultimately deny the EU citizens and their families the right to live and work in the UK or to claim benefits, although the Home Office has stressed that anybody found by immigration enforcement officers will have a 28-day grace period to submit an application.
The department has said that even then they will still be allowed to apply after the 28 days provided there are "reasonable grounds". Such grounds will include where a parent, guardian or council has failed to apply on behalf of a child or where a person has a serious medical condition preventing them from applying in time.
The Home Office said everyone who applied in time would receive a certificate of application that could be used to evidence their rights.
But it warned that would take time as postal applications would have to be opened, verified and the certificate issued. It said it would be accepting all postal applications in the next few days and all applications that were proved to have been sent before the June 30 cut-off.
Posts on social media revealed that many were still anxious about their futures. One woman wrote: "Am feeling extremely dispirited this afternoon. I have had a good cry." She had moved from Glasgow to Northern Ireland after finishing a higher national qualification in journalism and said she was "struggling to even get registered with recruitment agencies".
She wrote: "A lady asked for my immigration status on Monday. I sent her both the Home Office letter and a screenshot of what appears on the system. (Previously I offered to send the [digital] share code but she refused saying she did not know what it was and it did not sound legit.)
"Today she came back saying this is not enough [to] prove I can work in the UK … This is fifth recruitment agency I have interacted with and nobody seems to have a clue what settled status is."