The ONS has now admitted that its figures for net migration into the UK were underestimated for the best part of a decade. For the period 2001-2012 it has said that 346,000 more people came to the UK than under its previous count. That the migration figures are liable to revision should be no surprise as the underlying original data - the International Passenger Survey (IPS) - is (as the name suggests) only a survey. The UK Government doesn't properly count who comes and leaves. However, the underestimation is still startling.
Of the newly found 346,000 (the green line charted below) it is assumed that most of it can be accounted for by EU migration.
As we can see from the purple line net EU migration picked up after 2003 peaking in 2007 at over 100,000 per year. This coincides with the period of the majority of the ONS under-counting. The ONS believes its under-counting was due to a failure to recognise the large numbers of EU migrants coming through regional airports. Under the original IPS figures for the 1996-2011 period, a net figure of 3.9 million came into the UK of which 800,000 were from the EU. So who are the newly discovered 346,000 and how many EU migrants did actually come to the UK?
The answer is that we do not know. The 2011 census, which is a more accurate dataset, suggested that there were 2.7 million EU migrants in the UK of which 1.1 million had come from the 'new' post 2004 accession states. But this may itself be an underestimate and will include a large number of longer term migrants. It is likely that the ONS is right to suspect that their figures for EU migrants were underestimated but unless the UK starts counting people in and out we will never accurately know.
This is one of the biggest problems with the EU migration debate: the absence of reliable data and information erodes public trust in free movement. Yes, EU membership involves some loss of control over the border. But the UK is still out of the Schengen 'passport free' zone and other countries - via more effective identification (a complicated discussion in itself), taxation and border systems - are far better than the UK at counting.