An official report is set to spark fresh scrutiny of immigration in the EU referendum debate. Here we answer some of the key questions:
What is being published?
The Office for National Statistics will release an "information note" examining available figures on the difference between the registration of National Insurance numbers to adult overseas nationals and estimates of long-term international migration.
It is also expected to include new data on NI numbers actively being used.
What is the gap?
In the year to September, just under 655,000 NI numbers - needed to work in the UK - were registered to EU citizens.
But over the same period, immigration figures indicate 257,000 people arrived from the bloc.
What do we know about the NI figures?
There is no requirement when allocating an NI number to establish how long an individual intends to remain in the UK, so the numbers could include those who leave the UK after a short time, or who are here for a long period before applying for one.
And the immigration numbers?
The main official figures are for long-term immigration - meaning anyone who intends to stay in the UK for 12 months or more.
Estimates are based on data from the International Passenger Survey. Researchers carry out around 800,000 interviews each year as passengers enter or leave the UK by air, sea and tunnel routes.
What is the upshot?
The difference between the two sets of figures has fuelled claims that immigration may have been underestimated.
David Cameron previously described the figures as "quite complex" adding: "The reason why these numbers don't tally is, of course, you can get a National Insurance number for a very short-term visit and people who are already here but without a National Insurance number can apply for them."