A highly-anticipated official report examining figures at the centre of controversy over the scale of immigration from the EU will be released on Thursday in a potential flashpoint in the referendum campaign.
The Office for National Statistics will publish an "information note" on the difference between estimates of long-term international migration to Britain and the registration of National Insurance numbers to foreign nationals.
In the year to September, just under 655,000 NI numbers - which are needed to work in the UK - were registered to EU citizens.
Over the same period, immigration figures indicate 257,000 people arrived from the bloc.
The large gap has fuelled claims immigration may have been underestimated and scrutiny of the discrepancy has intensified in the run-up to next month's EU referendum.
Figures on the level of NI numbers registered to overseas nationals can include those who come to the UK for a short time before leaving, or people who have already been in the country for a long period before applying.
Long-term immigration figures only include arrivals over a 12-month period who intend to stay in the country for a year or more.
Last month a report from pressure group Migration Watch UK claimed net migration from the EU - the difference between the number of people arriving and leaving - could be running at nearly 50,000 higher than the official estimated level.
The ONS report is expected to include figures on short-term migration, as well as previously unpublished figures on NI numbers that are actively being used.
Ministers have faced intense pressure to release the data on active NI numbers being used by EU citizens in the UK.
Overall international net migration was estimated at 323,000 for the year ending September - well above the government's aim of less than 100,000.