On Thursday morning, the Office of National Statistics (ONS) will release an "information note" on the UK's migration figures.
Despite its innocuous official description, the release, which will attempt to clarify whether or not the UK's official migration statistics have underestimated how many people have entered the country, is likely to be a key moment in the EU referendum campaign.
Immigration is one of the two most important issues for voters – the other is the potential economic impact of Brexit – in the EU referendum, polling from Ipsos Mori shows, meaning the ONS intervention has real potential to swing votes.
What's this issue about?
The UK's official immigration figures are measured principally by counting people arriving at airports, at ports, and through the Channel Tunnel.
However, there's another set of statistics that counts the number of new national insurance numbers – which are required to work in the UK – issued to overseas nationals. These figures indicate a much higher level of immigration than the official count.
Initially, it was only academics – especially Jonathan Portes of the National Institute of Economic and Social Research – who puzzled at the difference between the two figures, but in February the figures became the centre of a media storm after The Sun accused the government of a "cover-up" on its front page.
The government has previously refused to give more detailed figures or reveal the total number of EU migrants suggested by the national insurance numbers, fuelling the cover-up claims.
But, in a March statistical release the ONS promised to release a briefing "to help explain further why the two datasets are showing different trends", incorporating data for the first time from the Department of Work & Pensions and HM Revenue and Customs.
What do the official migration figures show?
Every quarter the ONS publishes migration statistics. The key number in these is the net migration figure, the difference between those coming to the UK to stay for at least a year, and those emigrating. This is the UK's official migration statistic, which the Conservative party pledged to reduce to less than 100,000.
The data is calculated using different survey and administrative sources, including migration flows from the international passenger survey and employment of overseas nationals in the labour force survey.
The latest figures, which covered the 12 months up to September 2015, estimated the net migration of EU nationals to be 172,000. The higher figure on The Sun's above front page is the total immigration of EU nationals, which was 257,000 – but this excludes EU nationals who left Britain.
The next release of these official statistics is due on 26 May, a few weeks before the referendum.
What about the national insurance figures?
National insurance number (cutely referred to as NINo) data tracks the number of people who register for a NINo, which is required to start a job in the UK. This includes anyone starting their first job after school or college, or when someone arrives in the UK to seek work.
The latest data shows 630,133 EU nationals registered for a NINo between January and December 2015.
This figure is more than double the official measures for both net migration and total migration of EU nationals for a similar period – which has led to the row, and the accusations of cover-up.
An ONS spokesperson told BuzzFeed News the note will include all available sources and an explanation of the difference between these.
Why are the two numbers so different?
The short answer to this question is that we don't know: the "information note" the ONS is releasing on Thursday will be its attempt to settle this question.
But it is important to note that NINo registrations are not a measure of the number of migrants in the country, because not everyone who comes will register to work (overseas students, for example, may never register).
Another factor is that the official migration figures only show workers who are planning to stay for more than a year, while NiNo registrations include people who only work for a short-term period of a couple of months.
Because of short-term workers, and because NiNo numbers last forever, the total figures include a number of people who have left the country. This means the total number of NINo registrations for EU nationals is obviously higher than the number of current EU migrants.
The key figure – and one that's not been published before – is “active” NINos: registrations linked to individuals who are currently paying tax and/or claiming benefits, which may provide a better comparison with official migration figures.
Why are these numbers so important for the EU referendum debate?
Early on in the run-up to the referendum the Leave campaign was keen to stress that immigration was just one of a number of issues it would campaign on to secure Brexit.
However the issue has rapidly moved to the centre of the campaign as world leaders and international bodies have warned Brexit would come at an economic cost to Britain.
On Monday, Boris Johnson – who has previously spoken out in favour of immigration – made the issue the centrepiece of his five questions to the Remain camp in a speech in London. Three of the former London mayor's five questions centred on the immigration issue:
- How can you possibly control EU immigration into this country?
The Living Wage is an excellent policy, but how will you stop it being a big pull factor for uncontrolled EU migration, given that it is far higher than minimum wages in other EU countries?
How will you prevent the European Court from interfering further in immigration, asylum, human rights, and all kinds of matters which have nothing to do with the so-called Single Market?
According to official statistics, there are around 3 million EU nationals living the UK. With the official Vote Leave camp, UKIP, and Leave.EU alike all focusing on immigration, any confirmation that the figure is an underestimate – or even any suggestion along those lines – will quickly be seized upon as a potential boost to the campaign.
As such, tomorrow's release, however dry and careful the ONS tries to make it, is likely to fuel political rows for weeks in the run-up to the 23 June referendum.
James Ball is a special correspondent for BuzzFeed News and is based in London. PGP: here
Contact James Ball at James.Ball@buzzfeed.com.
Alberto Nardelli is Europe editor for BuzzFeed News and is based in London.
Contact Alberto Nardelli at email@example.com.
Got a confidential tip? Submit it here.