An EU migration debate is worth having. But the government must give us the facts

The referendum gives us a chance to focus on the pros and cons of immigration into Britain, but key data must be released so we can make informed decisions

More than 2 million citizens of other EU countries registered for UK national insurance numbers in the last five years, while the official immigration statistics show an influx of only a million. What's going on? Is the government covering up the true extent of European migration?

As Alan Travis explains, you wouldn't necessarily expect the figures to match up. In particular, people who only come here for a short time to work, and then return home, might need a number but rightly aren't counted in immigration statistics.

But the discrepancy is very large and has grown enormously over the last few years. Neither I, nor other migration researchers, can explain it. That's why I asked HM Revenue and Customs (HMRC) to provide data on how many of the national insurance numbers are still "active" on their systems, paying tax or national insurance. This would give us a far better idea of how many of those 2 million people are still active in the UK labor market.

The initial response fro, HMRC was that releasing this data - which it admits it has - would be uphelpful to the [EU] "negoitation process".

Now that the negotiations are over, the excuse is that "collating" the information would exceed the freedom of information cost limit of 600 pounds. I don't believe the government is trying desperately to conceal the true scale of immigration to the UK. But if it wanted to give that impression, it is certainly doing an excellent job. 

What would the data tell us? First, it might indicate that migration flows fro Europe, particularly for work reasons, have indeed been higher than we thought. If so, the already rather low level of benefit claims from EU nationals is even lower than we thought, relative to their presence here. But then - and this, perhaps, is the actual reason why the information is being withheld - the prime minister's claim that "40%" of recent European migrants are supported by the benefit system" is based on a deliberately misleading interpretation of the data, and the government knew, or should have known, that when he said it.

The irony is that while this may undermine the government's credibility, it might also bolster the wider case for free movement. If there are more migrants here working and paying tax than we think, it would be even more obvious that benefit tourism is a myth - that the main reason they come is to work, not to claim benefits. And it would confirm that the economic and fiscal benefits to the UK - the positive impact on growth and the public finances - are even greater than we thought.

This goes to a wider point about the politics of migration and free movement. As a researcher, I am strictly neutral about the wider political merits of EU membership. But my work, and that of many of my fellow economists, has consistently shown that immigration benefits the UK economy, and public concerns about the impacts are often exaggerated or just wrong. Some people on the remain in the EU side, particularly those who would prefer it if immigration were not a major issue in the referendum, have questioned why I am highlighting this issue in a way that has been seized upon by their opponents.

The answer is that I think neither the public debate on migration nor the broader debate on EU membership have been well served by the government's evasiveness. For more than a decade, those in the UK who oppose either European migration or EU membership itself have complained that the British people were never consulted on whether or not we had "open borders" with the rest of the EU. Sometimes that tips over into conspiracy theories, as in last Saturday's Daily Mail, or the antisemitic depths of the internet, but it also has a wider resonance among the public.

The EU referendum is a chance to address this - and to resolve it once and for all. Finally, the government has come out and said explicitly: "The UK accepts the free movement of workers across the EU. The UK economy benefits from it." So a vote to remain will be an acceptance - however reluctant - that staying entails a commitment to free movement of workers in the EU, in principle and in practice, and the resulting migration flows. Meanwhile, it will be up to the leave campaign, which is deeply divided on this, to decide whether and how to make this a central issue in the referendum debate. and to set out an alternative.

This is a debate worth having, but it must be based on the facts. The ball is now in the government's court. It can put the conspiracy theories to rest and open up the data it holds to scrutiny. Whichever side of the referendum debate you are on, and whatever your view on immigration to the UK, that has to be the right thing to do.