Immigration statistics sound like a dry subject - and they ought to be one. However, anyone who regularly writes or comments about immigration in the press is aware that there is a small but vocal minority who think that the government has been lying to us for years about the true scale of immigration, that the country is awash with millions of uncounted illegal immigrants, etc. Of course, this is nonsense. There is also a much larger group who, while not sharing this paranoia, don't trust government numbers on this topic.

Until recently, I would have been very wary of endorsing even the latter view. However, as I noted recently, there has been an increasing divergence between the official immigration and population statistics and the administrative data held by the DWP on the numbers of National Insurance numbers issued. Here's the chart again:

So I asked DWP and HMRC to tell me, using the same computer systems from which this data comes (which holds information on national insurance, benefits, tax credits and PAYE) to tell me how many National Insurance Numbers issued to recent migrants were "active" - ie showed recent payments of tax or NI, or benefit claims. This would give us a much better idea of the accuracy of the official statistics, which, using only survey data, estimate how many EU migrants have migrated here and how many are in the workforce.

Astonishingly, HMRC have refused to give me this data. Not because they don't have it. Not even because it would be too much work. But because, in their words

However, following the General Election, there is an active negotiation process at an international level in which UK Ministers and officials are engaged to secure support from the European Commission and other Member States for changes in EU law governing EU migrants’ access to benefits in the UK, in line with the Government’s manifesto commitments. The information is being used to inform the development of policy options as part of the negotiation process and therefore relates to the formulation of Government policy. HMRC continues to believe that releasing information in the form requested would, at this stage, be unhelpful to the negotiation process.

But, as the request shows (and HMRC don't dispute) I have not asked for any information whatsoever relating to the government's policy proposals or its negotiating position, whether to do with migrants' entitlement to benefits or anything else. I simply asked for some numbers on the recent migrant population in the UK. How would releasing this information in any way be harmful to the negotiation process? Does the government really believe that it is easier to formulate and negotiate policy if it conceals factual information from the public?

What would the data show? My current expectation is that it would reveal there are actually considerably more such recent migrants than the official immigration or labour market statistics actually suggest. But I don't know that and I'm quite willing to be proved wrong. But I do know that when the government has in its possession information on a topic of considerable public interest - the resident migrant population in the UK - and refuses to release it, it feeds precisely the type of paranoia and mistrust in official statistics I described in the first paragraph above.