In their report the ONS admitted their work “is not complete” and they will conduct further probes into migration figures over the next two years.
Glen Watson, Deputy National Statistician for Population and Public Policy at the ONS, 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.”
But Dr Carlos Vargas-Silva, senior researcher at the Migration Observatory at the University of Oxford, said long-term migration estimates “fail to capture a range of potentially important changes in migration trends”.
He added: “To provide a more complete picture we would need to monitor a range of indicators that include short-term migration flows.
“The measures that are currently in place to measure international migration all have limitations, so none should be taken as precise on their own – it is always important that we compare several sets of data to create a realistic picture of what is going on.”
The ONS report was due to be published at the end of this month – just weeks before the EU referendum.
But it was brought forward to today despite clashing with major Government proposals about the future of the BBC.