A total of 917,000 foreign nationals registered for a national insurance number in the 12 months to June, official figures show.
The figure was almost two thirds (62%) higher than the same period last year.
Three quarters of the total registrations - 697,000 - were for citizens from EU citizens - an increase of 66% compared with the previous year.
Romanians accounted for the most, with 170,000, followed by Poles with 128,000. There were 44,000 for Bulgarians.
The most common nationality allocated an NI number from outside the EU was Indian, with 40,000, the figures from the Office for National Statistics show.
One of the first Romanian migrants to arrive in the UK, after curbs preventing them from working were lifted, has spoken about his new life.
Levels of immigration into the UK could be about to hit their highest ever point, as the latest figures are due to be released this morning.