ONS has published the findings of work carried out to explain the differences between the number of long term migrants entering the UK, measured by the International Passenger Survey (IPS), and the numbers of non-UK nationals registering for National Insurance Numbers (NINos).

Using a range of administrative and survey data, we have analysed the reasons why the number of NINos being registered has been higher than the number of people estimated as migrating to the UK, and why in recent periods the gap between the two figures has grown.

The key findings are:

Short term migration (between 1-12 months) from the EU for work and study has been growing and largely accounts for the recent differences between the numbers of long-term migrants (over 12 months) and NINo registrations for EU citizens.

The International Passenger Survey continues to be the best source of information for measuring long-term international migration.

NINo registrations are not a good measure of long term migration trends, as they do not necessarily indicate the presence of an individual in the country, or how long they spend here.

The report notes that differences between NINo registrations to the ‘EU2’ countries – Romania and Bulgaria – appear to be particularly high in the two years since migration restrictions on these countries were lifted. However, a significant proportion of those people issued with a National Insurance number have since been active on our tax and benefit systems for less than 12 months, suggesting that the difference between these two figures can be largely attributed to short-term migration of 12 months or less.

Glen Watson, Deputy National Statistician for Population and Public Policy, said:

“We are confident the International Passenger Survey remains the best available way of measuring long-term migration to the UK.

National Insurance number registrations are not a good indicator of long term migration. This research shows that many people who register for National Insurance stay in the UK for less than a year, which is the minimum stay for a long-term migrant according to the internationally-recognised definition.

National Insurance numbers registrations do, however, provide a valuable source of information to highlight emerging trends. The number of short term migrants coming to the UK to work or study has been rising recently, but you need to consider the short term migrants leaving these shores as well to get the full picture.”